Sometimes, usually after almost getting killed again, Robin had to wonder why he still stuck around New York. Every survival instinct that he'd managed to scrap together over the millennia screamed at him to run, run as fast and as far as he could. Instead of doing the sensible thing though, he stayed.
He knew that Caliban probably thought he was staying around because of Niko. But despite what he claimed Niko wasn't the first person he'd ever fallen in love with who didn't return his feelings. He was more then capable of living with unrequited love, it just wasn't any fun.
Niko had chosen not to question why he stayed, although Robin knows that he wonders sometimes. For Niko though he was someone willing to go as far as necessary for Cal and that was more then enough. Even if everyone (and occasionally Robin himself) assumed his willingness to put himself in harms way to protect Caliban was because of his love for Niko.
Strangely enough it was Promise who probably understood him best. He thinks it's one of the reasons she puts up with him when she'd probably rather fling him out of her penthouse window. Going strictly by age she's still a baby in comparison, but she understands loneliness. She understands the need to belong to a group.
To form a family. It makes him laugh because if they're a family then it's one of the more dysfunctional ones he's had the opportunity to be part of, but it works. It's nice to have people care about him again. To care about the puck Robin and not whatever persona he's put on in this life time. To actually worry about whether or not he's alive. That kind of caring is addicting and even if it comes in a package that will probably get him killed he can't bring himself to leave it.