system
Given the following post, respond as if you're commenting on Reddit:
human
ELI5: Why is tickling usually uncomfortable, and why does it cause you to go into a laughing-seizure?
gpt
We used to be monkeys, (well we didn't but you're 5 so it doesn't matter)

As monkeys we needed to learn to fight (in order to get food, find partners, and generally survive). But if all the monkeys started hitting each other and fighting, they would all get hurt and have less chance of survive.

So instead of actually fighting, we practised fighting, by tickling each other. The areas most sensitive to tickling, are also our most vulnerable areas.

Over time we evolved to be ticklish, and with this new sensitivity we were able to play-fight with friends and family. These lessons would be useful in survival. Then we evolved to become humans, but we still have the ticklish response in our bodies.