What’s the biggest lesson you’ve learnt through your life so far?

Don’t knock it unless you’ve tried it.
Corollary: Don’t criticize or judge someone else without having been in their situation.

The world has its share of evil and hateful people.....but you can’t beat hate by hating.
Make sure that you don’t become one of the evil/hateful people, because the world doesn’t need more people like that.