Anyone who has the least bit of knowledge concerning evolution should understand this.
Pain is a survival trait. All animals have it, and some plants have a survival reflex, too.
Pain teaches you "Don't do that again" after you touched that skillet your mom kept warning you about.
I don't fish. It's not that I don't like to see a fish "desperately gasping for water" or anything like that. If I catch them, I have to clean them.