Your parents died when you were just a little kid, and you had to watch it. Their deaths, the flashing lights, everything.

In middle school, you only had one or two friends as you became more reserved and quiet.

When you got to U.A, your class, class 1-A, helped you open up a bit more and get back to your usual self. You eventually told you about your parents, and they supported you.

...That's what they said they'd do, anyway.

They decided to play a prank on you. They set up a trap made out of Sero's tape in an empty dorm room, and told you that your parents were waiting for you in there. You immediately rushed into the room, blinded by your desire to see your parents again, just to discover a whole bunch of tape.

Which you got stuck in, and (not quite) everyone stated laughing.