It's because he thinks poorly of himself. He doesn't believe anyone would ever love him so he kind of acts big and talks big. He's a marshmallow underneath, when he opens up
Yah see I'd rather have a character use Sarcasm (morrigan) or self depreciating humor (alistair) to cover up their insecurities, not sexual advances. As a romance character of course, but that's my personal opinion.