What does it mean when a guy tells a girl she's "such a sweetheart."?

I've never really really liked a guy, but I like this one. I'm usually not nice to guys either, but I decided to be with this one. We met on a dating site, and have been texting a lot til we meet really soon. I'll say nice things to him, compliment him, and he'll respond with "awww such a sweetheart" or "such an angel" and I can't tell if it's like a positive thing, or kind of waving away the things I say.

I don't want to be too nice to where it's lame and he starts to only see me as a friend or doormat you know?

What do you guys think?