Love at first sight.?

There have been a few times when a woman I have been dating asks me "Do you believe in love at first sight". Personally I think it is possible although I have yet to experience it. Now my question is this, if these women feel this way about me, assuming that's why they asked, then why is it that when I start to build up feelings for them **** hits the fan? I think it's human nature to want what we can't have then when we do have it, it really doesn't matter anymore. What do you think?