It's not things that make us happy or sad that we find, but it's things that we care about that we find, and the things we care about can make us happy if they go right, and sad if they don't, and the things we don't care about don't really effect us
maybe that's why depressed people stop caring about things, sort of as a defense mechanism in fear of things going bad, like it's sometimes easier to feel dull and empty than be hurt all the time because things they care about aren't going well