For them I would probably suggest something that is almost as impossible: You are what you practise, so practise being optimistic; practise having fun even if it doesn't feel like much fun. Laugh even if it makes a hollow sound. And remember that your friends do care about you even when that awful little voice in your head says you are unloved and unlovable.
For the rest of you who are just living life and are not really depressed ... just a little down, here is another one of those cheery quotes that are infuriating when you are depressed but actually true when it is possible to follow the trite advice contained in them:
"Things turn out best for the people who make the best of the way things turn out."
The problem with cliches is that they are cliches precisely because they are so true. That's why they have been repeated so often.