Why does every female i like only want to be my friend, nothing more?

This has happened to me soo much in recent years. I started to hang with a girl who I really liked. I took her out showed her a good time, and told her how I felt and she could only giggle. We have shared many times together out at all sorts of romantic places normaly resulting in kissing and a goodnight. She told me she had feelings for me, but couldn't be with me(reason was left out). This is just one of many occations, but its the one that hurts the most. I have plenty of friends and I'm looking for a significant other not another friend, and it kills me seeing someone I like go for some other person. I can't help my feelings, but they are always unrequited. Aam I to nice a guy? Should I just go cave man and claim one?