Don't criticize, complain or condem

This is the first principle of Dale Carnegie's how to win friends and influence people; I took a communication skills' training at work.
Anyhow, the point is that I really love this principle; it should be used on people, but I'd rather use it in life. I mean, wouldn't life be much easier if we were all satisfied with what we have, as we strive for something better? Imagine the amount of friendships that wouldn't be ruined, as well as business partnerships and even marriages. If we go through life content (not happy), things will differ completely.
I personally am trying to limit my complaining, because it's really defeating and repulsing. I guess the hardest point is not to criticize, because I always do. I criticize life, myself and the things that I am not doing (that I should). The whole thing helps to be optimistic, a word that escaped my mind for the past 5 years or sth.

So, here's to optimism, may it be easily obtained and preciously contained.