You really must enjoy moments of happiness. You are going to miss them when they are gone away. Those moments with friends, family, trips, happy memories, when you enjoy something, when you love somebody, when you get all what you want. But when all this happy things happens, you have to understand that maybe this is not forever. There are moments that they are not so beautiful, great or happy. You have to understand this, when you understood this, you will be happy.