A boy once talked and laughed with me, but one day he hated me, which puzzled me. Who can tell me why?

You have to see first, does he hate others as much? If so, then he may be very upset and bad that day.

If it's just for you, there must be a reason. You have to believe that a person is angry or hates a person, not for no reason, there must be a misunderstanding.

You should find out the reason why he hates it first, and then think of something. Now is not the right time for you to ask him. He hates you, so you should avoid him. Time will dilute everything. I believe men don't hold grudges too much, no matter whose fault it is.