Gemara

(R. Chelbo): Anyone who benefits from a wedding feast and does not gladden the Chasan transgresses five Kolos (voices) with which Hash-m blessed Yisrael -- "Kol Sason v'Kol Simchah Kol Chasan v'Kol Kalah Kol Omrim Hodu Es Hash-m Tzevakos."

Question: If he gladdened the Chasan, what is his reward?

Answer #1 (R. Yehoshua ben Levi): He merits Torah, which was given with five Kolos -- "va'Yhi va'Yom ha'Shelishi... Kolos u'Verakim... v'Kol Shofar... va'Yehi Kol ha'Shofar... veha'Elokim Ya'anenu v'Kol."

Answer #2 (R. Avahu): It is as if he offered a Korban Todah -- (the verse "Kol Sason v'Kol Simchah..." continues) "Mevi'im Todah Beis Hash-m."

Answer #3 (Rav Nachman bar Yitzchak): It is as if he rebuilt a ruin of Yerushalayim -- "Ki Ashiv Es Shevus ha'Aretz keva'Rishonah."

Kesuvos 15b (Mishnah): If witnesses say that a Kalah had a Hinuma (a dome of myrtle or a veil) and uncovered hair at the Chupah, her Kesuvah is 200.

17a (Beraisa - Beis Shamai): When dancing in front of the Kalah, praise her like she is;

Beis Hillel say, we say 'she is pretty and Chasudah (has grace).'

Beis Shamai: If she is lame or blind, do we say that she is pretty and Chasudah?! "Stay far from a false matter"!

Beis Hillel: Don't you agree that when someone buys something, we praise what he bought?!

From here we learn that a person should always be amiable.

R. Yehudah used to take a myrtle branch and dance in front of the Kalah, and say 'the Kalah is pretty and Chasudah.'

Rav Acha would carry the Kalah on his shoulders and dance.

Rabanan: May we do similarly?

Rav Acha: You may, but only if she is like a log to you.

(R. Shmuel bar Nachmani): One may look at the Kalah all seven days, to endear her to the Chasan.

The Halachah does not follow him.

(Beraisa): We cease learning Torah to escort a Mes or a Kalah. R. Yehudah used to do so.

Rishonim

Rosh (Kesuvos 2:3): Some say that the Halachah does not follow R. Shmuel bar Nachmani, who permits looking at the Kalah all seven days, but the first day, which is the primary time to endear her to the Chasan, it is permitted. If not, how can people testify that she went out with Hinuma and uncovered hair? This is wrong. One may not look at her even for a moment. Rather, since they see the Hinuma that they made, and they say 'this is for Plonis', this is testimony. Alternatively, seeing the Hinuma on her or her uncovered hair is not looking at her face.

Rosh (5): Some say that one may be Mevatel Torah to escort a Mes or a Kalah. They support this from Megilah 3b, which says that a Mes Mitzvah overrides Talmud Torah. This implies that for a Stam Mes, there is no Chiyuv to be Mevatel Torah. It is Reshus (optional). However, the Gemara connotes that it is a Chiyuv.

Tur (EH 65): The Medrash says that dogs did not eat the feet of Izevel because she used them to dance in front of the Chasan and Kalah.

Tosfos (Kesuvos 17a DH Kalah): Beis Shamai holds that if she has a blemish, one is silent, or he praises something nice about her, e.g. her eyes or hands if they are nice. Beis Hillel says that one totally praises her, for when one mentions what is praiseworthy, this implies that the rest is lowly.

Poskim

Shulchan Aruch (EH 65:1): It is a Mitzvah to gladden the Chasan and Kalah and to dance in front of her and to say that she is pretty and Chasudah (bears grace), even if she is not pretty.

Prishah (1) and Beis Shmuel (2): Even though it says "mi'Devar Sheker Tirchak", we can say that he means that she is beautiful in her deeds.

Chelkas Mechokek (1): If she has a blemish, one does not say that she does not have a blemish, for that is total Sheker. Rather, he says Stam that she is pretty and a string of Chesed is over her, unlike Beis Shamai.

Prishah (2): One who did not benefit from the wedding feast is not obligated to gladden the Chasan.

Beis Shmuel (1): The Poskim connote that it is a Mitzvah to gladden them [even if he did not benefit from the meal].

Sefer ha'Makneh (Kuntres Acharon, 65:1): It is a Mitzvah for anyone to gladden them, even if he did not benefit! What is the meaning of "he transgresses five Kolos"? After the first day, we bless only Asher Bara (and Borei Peri ha'Gafen), unless there are Panim Chodoshos. Then, we bless all the Sheva Berachos. The Rosh explains that Panim Chodoshos are people for whom we increase the meal. The Chelkas Mechokek and Beis Shmuel (62:13, 62:10) bring opinions that it suffices that it is proper to increase for them, even if they did not actually increase. We already blessed Sheva Berachos at the Chupah. We repeat (the other five) after the meal due to the Simchah of the Chasan and Kalah. Therefore, those who eat with them must increase their Simchah, to justify the extra five Berachos, which were enacted corresponding to the five Kolos in the verse. Therefore, one who does not gladden them makes the Berachos l'Vatalah, and transgresses the five Kolos.

Ezer mi'Tziyon (1): One fulfills gladdening the Chasan if he says one of the Sheva Berachos or (leads) Birkas ha'Mazon, or converses with the Chasan to arouse him to Simchah of Mitzvah. An esteemed person fulfills it through going to the meal, just like we find that a woman can become Mekudeshes through the Hana'ah she gets that he accepted a gift from her. One fulfills it through passing food or drink to the Chasan, dancing in front of the Kalah, or saying a joke that brings the Chasan to laugh or to visible Simchah, or praises the Kalah in front of him.

Rema: We find that R. Yehudah would dance in front of the Kalah. We are Mevatel Talmud Torah to escort the Kalah to the Chupah.

Bach (1): Even though the Talmud Torah is equal to all the Mitzvos, we are Mevatel Talmud Torah to escort the Kalah to the Chupah. This is not only for a Stam Chacham. It is even for a Gedolei ha'Dor, like R. Yehudah did. The wicked Izevel was punished, except for her feet. This shows that it is a great Mitzvah.

Chelkas Mechokek (2): The Rosh says that it is a Chiyuv to be Mevatel Torah. Perhaps only when one sees her enter the Chupah, he must honor them. If he knows that there is a Chupah in the city, he need not neglect his learning to go there.

Beis Shmuel (3): The Beraisa connotes unlike the Chelkas Mechokek. It equates Hachnasas Kalah to Hotza'as ha'Mes. Just like the latter is a Chiyuv even if he does not see the Mes, similarly Hachnasas Kalah.

Toras Chayim (Avodah Zarah 17a DH a'Bei, brought in Pischei Teshuvah 2): One may not dance in a circle with the Kalah during the seven days of Mishteh, even if he does not hold her hand, just they hold onto the same cloth, like some Chachamim do nowadays. The Gemara forbids any closeness. We do not distinguish a Kalah from other women. Nowadays, no one can say 'she is like a log to me.' Chachamim said 'how do we dance in front of the Kalah, not with the Kalah. One may not look at her face, just like one may not look at other women. Those who dance with her will be punished in Gehinom. Anyone who can stop them must protest.

Be'er Heitev (2): Honor of Chasan and Kalah exceeds honor of festivals. Therefore, if one has a new garment to wear at the Chupah, and a festival came beforehand, he should not wear it, and leave it for the Chupah (Maharil, Nisu'in 10).

Pischei Teshuvah (3): This requires investigation to resolve this with Be'er Heitev OC 90:11 [who says that if a prisoner is permitted to pray one day with a Minyan, he should not wait for Yom Kipur. Rather, he prays with a Minyan the first day possible.]

Shulchan Aruch (2): One may not look at the Kalah.

Beis Yosef (DH umi'Kol): The Tur explains that even though it is a Mitzvah to gladden the Chasan and Kalah and to dance in front of her... do not say that likewise, one may look at her face, for this will endear her to the Chasan, and this is the ultimate Simchah for the Chasan and Kalah. This is wrong. One may not look at her.

Rema: However, one may look at ornaments on her, or her uncovered hair.

Drishah (5): What is the Tur's source to permit looking at her ornaments? One may not look at other women's ornaments! The Tur (21) forbids looking at colored clothing of a woman whom he recognizes. The Rosh permits only the Hinuma and her uncovered hair. Perhaps he learns from the Rosh's Heter, which is because he does not look at her face. Initially the Rosh wrote that one may not look at Hinuma when it is on her, but he concluded that it is permitted, for he does not look at her face. The same applies to all ornaments. Chachamim were slightly lenient, in order to endear her to the Chasan. They were not lenient in order that people will be able to testify that she was a virgin. The lenient opinion in the Rosh inferred from the fact that such testimony enables her to get a Kesuvah of 200. We do not discuss Resha'im who looked improperly! He gave a different reason (for the lenient opinion), in order to endear her to the Chasan.