While I'm not sure this will apply to all, it definitely applies to me. Ok, so it's my secret.
As an A personality, a perfectionist and a very "anal"itical person, I've always been very driven, but I've also realized that this way of being has made me very tired, always looking for something (better) and never completely happy with anything. Well, that's no way to be (if you want to be happy).
So I started to change my outlook on things, and while it wasn't easy, having a happiness goal at the end of the line helped tremendously.
Instead of looking for the "best" I started looking for something "good". And let me tell you, finding good is a lot easier than finding the best! Especially in our modern society, where we have so many options on anything, finding the best intrinsically means you need to evaluate every option, and there lies the problem: do you really want to spend your life looking for the best of everything and being left with no time to enjoy life itself? Me, neither.
Why having to try out every body soap ever created (read my previous post) to find a favorite one? What if you find that you really like the third one you've ever tried and there are still 10 different brands you haven't tried on the shelves? Why spend the money and time to try out the other 10 and then probably find out that you still prefer that third one? Be content. Appreciate that third one and move on.
The body soaps example is just that, an example. You can apply it to many other things in life, from big to small: a couch, a house, a dog, a job, a man...
Just don't confuse being content with settling. When you settle you're rarely happy, and you'll settle mostly because you ran out of choices (or you thought you had no choice). When you are content with something, it genuinely makes you happy (if it doesn't, then you're settling), it's a choice you can stand behind and it fills that void that kept you looking for more.
And the most important place to be content is with yourself. Appreciate what you have, learn to love what you don't like about yourself, and be content with who you are. Sometimes is not so easy, but with time and effort and a little self-love, the minute you are finally content with yourself you'll realize it will be a lot easier to also be happy.