Han Solo's name says it all. (Well, not his first name. That doesn't really say anything.) We're talking about the fact that Han Solo is a textbook loner. He may be charming and quick with a witty one-liner, but he's always looking out for #1—until Return of the Jedi, that is.
Return of the Jedi sees a kinder, gentler Han. Gone is the selfish dude from the previous installments in the Star Wars saga who was scared of commitment and motivated by money. When he's unfrozen and freed from the carbonite in which Jabba had imprisoned him, he doesn't just warm up physically; he warms up socially and emotionally.
Instead of toying with Leia, he tells her loves her, and he forges a real friendship with Luke. Check out this exchange—there's no way that Han of Star Wars would have been so generous or open:
HAN: I'm sure Luke wasn't on that thing when it blew.
LEIA: He wasn't. I can feel it.
HAN: You love him, don't you?
HAN: All right. I understand. Fine. When he comes back, I won't get in the way.
LEIA: Oh, it's not like that at all. He's my brother.
It takes a serious amount of zen to say "Yeah, if you love someone else, that's okay." Han also takes on more responsibility with the Rebel Alliance, accepting a position as a general and leading the team on Endor. He even loans out his precious Millennium Falcon to Lando.
Maybe all of these positive changes are just side effects of hibernation sickness, but probably not. In Return of the Jedi, Han finally grows up. He commits to his friends, he commits to his girlfriend, and he commits to a cause greater than himself. In short, he's solo no more.