Is it normal to always love someone even when that person keeps rejecting me?

Normal, but not advisable. Focus on healing. Don't waste your time.

How do you turn deep emotions on and off?

I think you are wasting your time.

There was this one girl in high-school I wanted so bad, and so did a pal of mine from back then. We agreed that no matter who she ends up with, we're cool. She chose him. Two weeks later, they broke up after she saw him with another girl. And I was still not over her.

Point is, I wasted so much of my teen years "loving" someone who didn't feel the same.

As others have said, don't waste your time.

Move on

I don't know if it's normal, but it's certainly idiotic.

It's pretty normal, because as someone else said, you can't really turn your emotions on or off and you don't really all of a sudden stop loving somebody, it takes time. However, it is a waste of time to keep trying with them and instead, it's healthiest to put that energy towards healing and moving on.

I know rejection hurts really badly and it's hard to get over people, but you will get there eventually and it will be okay!

If they keep rejecting you, find someone else. You're only going to keep hurting yourself if you keep trying to pursue what you will never have.