Why can't everyone just say yes?

So, today I had a pretty good day and I'm getting farther into this relationship with this amazing girl.  But, I also wanted to ask the FES girl to prom for she is very beautiful.  She said she was unsure if she was going to go to prom, but i was like you definitely have to go no matter what.  I don't mind if she doesn't want to go to prom with me because i can easily ask the girl I am trying to hook up with and shes a definite yes. But its like, the person you go to prom with doesn't really matter and the part that matters is the person you dance with at prom. (Just a side note, I am a pretty laid back dude that lets everything slip off my shoulders for life can not be taken to seriously, otherwise you will never actually have fun in life.  You need to simply "let the good times roll" and that's the way i live my life, live for the future forget the past, and always remember that when you do something, never regret it and always follow through with what you say.) Getting back to the topic at hand, why do so many people say no and not yes, people say that you never have to do anything that can harm you if you say no, but if you are the one to say yes, then you are the one that is going to make things happen.  Ehh, but even if i don't get a yes from the beautiful FES, i can always ask my good neighbor A to prom for she is also looking for a date and she is a cute girl with a great personality whom i have known for my whole life.  And if worse comes to worse, i can always ask lil C from a couple towns over for a definite yes because she is a freshman and never has been to prom before.  But then again, I could ask the girl I talked about at the beginning whom i am going on a date with her on friday, and if all goes well, she could be my girlfriend, so is it bad if i bring another girl for we may not still be in a relationship by prom. If you have any input please tell me what you think i should do.