Ok why is it when I look at a girl, all they do is laugh?

Sorry but when girls do that they are making fun of you............BUT don't worry if you want them to stop just laugh back, not sarcastically but kinda genuine cause if you make it sound sarcastic they will feel creeped out. Usually this happens when you get like someone upset or one of your enemies turn everyone on you. Now the most important thing is to not look upset or puzzled this would give them the satisfaction. Another thing is that they think that you are attracted to them and they are flattered and they do not know what else to do. Good luck, believe me the same thing happened to me, I did all this and it eventually stopped (like a week or 2) believe me. Oh and be really friendly!

Depends what sort of laugh it is. If it's a giggle then she's flirting. I mean what else do you do if a guy's staring at you? The normal reaction I'd say would be a small laugh.

Depends man, it could have been the clothes you were wearing, maybe it could have been how you walked, idk is the laugh like a giggle or a laugh like bust out laughing? Next time look at them approach them and simply say oh I'm sorry was something funny? If they make fun of you bc of what your wearing simply laugh at yourself to that way they know your comfortable making fun of yourself also, or say well then if you don't like what I am wearing maybe you should take me shopping with you, since you seem to have better taste, what would you suggest I wear to make me look good? Or it could just be your imagination they could have been laughing at a joke and you just happened to walk in at the same moment.

It could very well be your imagination. Girls can be extremely open when you ask them. Find one (even a sister, cousin, or mother) and ask her if you do anything that seems funny. It may even be that you're staring too long at them. Are any guys laughing? Is it a giggle or a belly laugh? Either way I doubt it's anything to worry about.