I never got any sense of how Lupin and Tonks worked anyway. What attracted her to him, what they had in common... wish Rowling had filled in just a little of that.
I think it’s first, because he was “exotic,” just as she was, and second, because he was needy. (And of course, he was kind, too.) She thought she could make him happy, but it’s clear in DH that he wasn’t going to let her do it.
That seems pretty obvious, really ... aside from his furry little problem, Lupin is thoughtful, understanding, wise, and generally a good guy. He keeps plugging along in very adverse circumstances. He's also got a bit of danger about him, both in what he does for the Order, and of course that he's a werewolf as well. He seems like the sort of guy that a lot of women are attracted to -- or say they are.
Lupin's attraction to Tonks ... well, guys are a lot more simple in that regard. She's attractive, smart, funny, able, and devoted to him.
Remus was just so dang sweet and vulnerable! I’m sure that’s why Tonks was attracted to him.