IF YOU ARE A STUDENT INSCHOOL:Well usually the boy would barley or not at all talk to you, he doesn't look at you that much or doesn't look at you at all and if you try to talk to him and he walks...
1 reason you will hate someone : they ruin your birthday but you know you hate them when you want to cry when you interact with the person you are very happy then the person says a few words to you...
Maybe they are jelous, maybe there is someone telling nasty lies/rumours about you. Maybe they are just plain nasty and want someone to bully, I hope whatever the cause is, it will end for you soon....