Sometimes people start rumors to make them feel better. It makes them feel better than you and other times it's cuz u did something that made them mad, and they wanted to get u back.
I think there' s a lot of reasons, ranging from boredom to jealousy to purposeless malignancy. You know you've truly matured when you realize you don't care about other people's tendency to say and believe total bull****. It's going to be OK. You can prove a rumor about you wrong by just being yourself.
To make the person feel inferior to them because they envy something of you. It really is just based on their insecurities, and actually think you may become more popular, may be more prettier, may have a guy/girl that likes you, etc.etc.
They just want to make you look bad, point out your flaws so people can't see their's.