Depends on the guy, when I met my boyfriend we were both in very low crappy positions in life.
And then just before and when we got together, he was continuously like; 'I don't deserve you' and 'You should be going after someone better'
I spend alot of my time even now (6 months later) convincing him that he is perfect for me, and that I don't want anyone else.
I don't think he still believes me wholly, but he's alot better than he was, I think because he now knows Im not going anywhere.
You need to give us a bit more of an idea of the situation.
Because thats just one idea, theres loads of different things.