Why do people always do this to me?

For some reason,people are usually nice to me at first and seem to like me.but then eventually,they become a lot meaner and are rude and just don't seem to care anymore.For instance,there's this guy in my homeroom.We didn't really know eachother too well.But he started talking a lot to me on facebook,and then in school.we talked every day and he was so nice to me! he was never rude and told me that if I was a little more social,he thought a lot of people would like me (I'm kinda quiet).during breaks (like in between periods and lunch and stuff),he would come to hang out with me.as we got closer,we would tease eachother but only in a friend way,nothing actually mean.then he started getting kinda mean,and we didn't talk as much.now he's rude to me all the time,we don't talk to eachother,he even called me a bi*ch last week.I didn't do anything mean to him,I swear.It just seems people do this kind of thing to me a lot.his friend was nice to me,and now he's rude to me too.what's wrong with me,why do they do this? D: