That's true for some people, but for others it just isn't. I can't choose to not stress over the fact that the tests I'm taking now decide whether I can go to college or not. I can't not stress over my deteriorating hearing. I can't help that I worry over my sister's problems.
My sister can't help worry about me. Not since she found me with a knife about to do what you described, there are bloodstains that will not come out. We don't bitch about our lives, we worry about them. Ameya, my sister, is having a hard time finding a job because of what she looks like.
I have a hard time keeping a job because of the urges to make everything as neat as possible. Sounds good at first, but trust me it gets old.
We're watching our mother get older and older, some even before her time thanks to our uncle-he-who-must-not-be-named. In short, just last month he was in a hit and run. He ran. He stole the poor victims wallet, but not before spitting on the corpse.
We're watching our father loose his memory, slowly.
I'm not trying to create a pity party or some kind of competition, but you can't just assume the problems of others can be solved by just choosing to be happy.