The most important lessons

So, I know you probably have a lot of important questions that you have to answer, so it's okay that you don't answer this one even if I'd like you to do so.
However as a massive fan of your books, and learning quite a lot from them and using them to grow as a person, I wanted to ask you a simple question:
What do you think are the most important life lessons you have learnt as you have grown older and wiser?  And what advice would you give to a young man in his early 20’s? 
(Julien Hoez、男性、21歳、Student Martial Artist)

The important lessons I learned when I was young? Most of them are very simple ones. The things you owe, you have to return. It is better for you to be injured than to injure somebody. Complaining and explaining solves absolutely nothing, etc.

Don’t you think they are simple and straight forward enough? But they work just fine over the long run. Try them.