Having friends. In Rapture he basically HAD no friends; there were the Little Sisters but that was a relationship defined by pheromones and brainwashing as much as anything else, and even the love he felt and feels for Eleanor was problematic in that sense. Now he has actual relationships with people he likes and cares for and that, probably more than anything else, makes him feel human again.

...Oh, God. That's a tough question. Once he would have absolutely said yes. Now I think he'd lean much more toward no, but he'd be tempted. His memories have caused him extraordinary pain without much in the way of an obvious upside yet, and while he loves Neil, he also feels like he's not much good for Neil and sometimes wonders if it might be better for Neil if they'd never gotten back together at all. So he'd probably say no, but probably not right away.