Why is Lev Chachamim in Beis Evel?
Rav Sadya Gaon: It is a place of rebuke.
Rashi: They think about the day of death.
Ibn Ezra: Above, it said that is it better to go to Beis ha'Evel than to Beis ha'Mishteh (2). Chachmei Lev, even when they do not go to Beis ha'Evel, Beis ha'Evel is in their hearts.
Rashbam: They desire to go there, for amidst [being there] they put the day of death to their hearts, and do not sin.
Ri Kara: There, they hear Divrei Chachamim. They see that this one died, and teach people to repent while they are still alive.
Seforno: Chachmah are sad over their lack, and worry about the future, in the days of payment [for people's deeds], and strive to acquire eternal perfection.
Metzudas David: They always think about mourning, to humble their hearts to guard from sin.
Why is Lev Kesilim in Beis Simchah?
Rav Sadya Gaon: It is a place of song and Simchah.
Ri Kara: Their hearts do not worry about evil that will come on the land (11:2), so they put to heart to go to Beis ha'Simchah.
Seforno: They are happy with pleasure which is only in the present, without seeing the consequence.
What is the consequence of "Lev Kesilim b'Veis Simchah"?
Rashi: They are not concerned for the day of death; their heart is strong (without worry) like the Ulam.
Rashbam: Amidst their Simchah they sin; they do not put Yir'as Hashem in front of their eyes.
Metzudas David: They always think about Simchah, dancing and light-headedness, which leads to sin.
