With Snape and Lupin: Snape likes to feel needed and to have a place (funny, since Lupin's always the one who "does like to be liked"...) which is a large part of what draws him to the werewolves and to Lupin. He does not feel, as it were, that love can be unconditional. Lupin is used to being dependent but not always happy about it, plus of course he has his own problems. Plus, consciously, neither of them entirely trusts the other. In his wolf-self, Lupin is able to be trusting and partially dependent without unhappiness, and also able to recognize that Snape will do almost anything for someone who depends on him. (Which is obviously perfectly canonical; and it gets him into more trouble.) Snape is able to feel sure that he actually is needed...Both Snape and Lupin have been forced into things a great deal and have real problems with the idea of freely choosing. This is why their conversations about the past and politics are so unhappy...although I would contend that they've come quite a long way in terms of what they're willing to say to each other and the kinds of responses those things elicit.
I really need to sit down and do a detailed outline for the next few chapters today....