Zlatan really loved his time in Juve. He doesn't love Juve, he doesn't care about Juve, but he is full of respect for what Juve used to be and it's strange coming from someone who shows respect to noone.
He was just a young kid making his first steps in the top class football and suddenly he found himself protected by two strong individuals, almost like father figures to him back then (Moggi and Capello) as well as a bunch of world class players who were all 5-10 years older than him (Nedved, Del Piero, Thuram, Cannavaro, Camo, Trez etc). It probably felt like a great dream. He was learning from the best.
Later he never had a chance to experience anything similar. In Inter he was the main man. In Barca and Milan he was an already established player, now he can even be considered a veteran. During his time in those clubs he was simply doing his profession. In Juve it was different. That's why he never says a single bad word about us and is full of praises.