A lot of being successful in life is achieving enough stability to be able to take a punch.
Life is like downhill skiing, there's a lot about the landscape and environment that you can't control, get used to it. But you also can navigate the slopes with quite a bit of agility within those constraints, especially if you've practiced and built up strong muscles.
Most people want to go as fast as possible towards their goals…career success, riches, world-travel, personal development, raising a family, whatever.
(Note: some people have taken themselves out of this game, gotten off the treadmill, the career ladder, the race for an idealized American dream…an admirable, but “judged”, lifestyle choice these days.)
So anyway, we're skiing. Here's the real kicker: life will take cheap shots. So now imagine while you're skiing on the edge, every once in a while when you're definitely not expecting it, life throws a punch. A big full-fisted hook to the face. A family member is killed in an accident. A loved one is diagnosed with cancer. Your home burns down. You get laid off unexpectedly from work. Tragedy and surprise side-swipes thousands every day.
When you take that punch, you're hopefully stable enough to absorb it and regain your balance before the next one comes. Otherwise, you're out for the count, careening down an icy slope, which either ends in a long unceremonious glide to a stop, or in a more sudden deceleration, courtesy of a tree or rock. And during this slide, you risk taking out lots of other skiers on the way, sharing your pain and sending them sliding out of control as well.
The reason you should work so hard and exercise those life-muscles so hard is so you can feel extra confident skiing down the slopes of life, and not allow yourself to be paralyzed by the perpetual fear of getting sucker punched.