You may be doing or saying something wrong or just saying it the wrong way. Perhaps you're giving off vibes you don't mean to be giving off. You may be coming off as desperate or too needy or you could be acting like a jerk (sorry-I'm not saying that's the case but it could be, I don't know) without even realizing it. It's not something you want to hear but it's a possibility you're inviting rejection by not checking what signals you're giving off.
Maybe it'll be better to hang back and act aloof. Smile and be friendly but don't let them know that you're interested in them romantically.
You have to try to look at yourself in third person if you can.
You can turn this into a learning experience. You just need to figure out what messages you're sending out-whether or not you want to be sending those messages out and then workin on changing those messages if they're not what you want people to read off you. If they are the messages you want people to be reading and they keep responding to you negatively-you probably have some things to think through or you're hanging out with the wrong people.
((((((((((AriesMars))))))))))))