Maybe it is the type of girls you are going for? Maybe try expanding your horizons and see if you click with other females that you generally don't think you would. There is also such a thing as being too nice (could come across as a bit desperate) and like Spring said trying too hard.
As for answering your question what makes you wanna say yes to certain people, and no to others? I think it is just a matter of feeling that connection with the person asking me out. You know pretty much straightaway if it is someone you want to get to know further.
Age has something to do with it too. I tend to go for the nicer guys now whereas back in my teen years I tended to like the dickheads. Don't know why... probably more a reflection on how I felt about myself more than anything else.
Some people are drains and some are radiators... Keep clear of the drains and hug the radiators!