Why's he being so nice?

If it makes you feel uncomfortable, then sit him down and let him know. He won't know how you feel unless you tell him. He's obviously trying to make you feel positive about yourself, and let you know what he thinks of you. You should be glad you have someone who thinks of you in that way. So, the best advice is, enjoy it, savour it - and just let him know that he doesn't need to say it as much.