Why is Iveles happiness to a Chasar Lev?
R. Yonah: Simchah is aroused from the heart. One whose Simchah is from Iveles, he is called Chasar Lev.
Malbim: Chasar Lev has no power that rules to force him to go in a known way. Also in the way that he goes, he is Chasar Lev. "B'Sifsei Navon Timatzei Chachmah v'Shevet l'Gev Chasar Lev" (10:13) - he is like an animal, which has no internal conductor. It needs an external conductor to guide it in the path - a staff. He is the opposite of a Navon, in whose lips you find Chachmah; it guides him. When a Chasar Lev finds Iveles and doubts, he rejoices. Also Evilim are Chasar Lev in the end - "ve'Evilim ba'Chasar Lev Yamusu" (10:21).
Why does it say that an Ish Tevunah goes straight?
R. Yonah: Ish Tevunah is not the exact opposite of Chasar Lev 1 . Not all Nevonim rejoice in Mitzvos to the point that they reach the level "Simchu va'Shem v'Gilu Tzadikim" (Tehilim 32:11). Every day he straightens his Midos, "Ki Yetzer Lev ha'Adam Ra mi'Ne'urav" (Bereishis 8:21). He must fix it and straighten it and put his desires towards proper matters. Fixing Midos is the great fence from Iveles and sin, and a tall ladder to reach Avodas Hashem, His love and desire. How great is the distance between Ish Tevunah and Iveles, the Midah of Chasar Lev. Not only does a Chasar Lev not guard from Iveles and sins - the straight path of fixing Midos - he also does Iveles, and does not fear lest his Yetzer seize him to make him sin. He rejoices to do Iveles!
Malbim: An Ish Tevunah, he knows the straight path via Binah (straightness), and he goes straightly. He does not need a teacher and signs on the road - the signposts of Chachmah, for he goes straightly due to his Binah, and does not err.
R. Yonah: When this Sefer mentions two Midos in one verse, one beloved and one hated, they are opposites.
