After achieving of the traditional markers and success, becoming financially independent, etc., my lesson:

Life is all upside.

Even pain — pain is not what makes life a downside. In fact, it is what makes life enjoyable.

If you never felt pain in life, you will not appreciate the good things.

Also from a physiological perspective, pain is actually an necessary ingredient for growth. Also, childbirth would be impossible without pain, as pain is the signal to the wounds body to push out the baby.