Joy and gratitude are intertwined, and finding the things for which you are thankful is the fastest way I know of to move into joy. And from this state, it is easier to create those things you most desire because your current attitude attracts like energy. So if you focus on things for which you are grateful, you attract more things to be thankful for.
Nobody ever got happier from grousing about the things in life they didn't like, and it's pretty difficult to feel joy if all you're thinking about is how bad things are. Fortunately, everyone has some aspects of their life for which they can be thankful. I do realize some days you have to search a little harder, but those things are always there. And when you focus on the good parts (no matter how small), your attention shifts away from the elements in your life that are currently making you unhappy. Why is that important? Because where you focus your attention attracts similar energies, so if you are focusing on something negative, you attract more negativity. If you are focusing on something positive, you attract more of the same.
Joy is a choice and gratitude is one of its triggers. It's up to you to choose joy when you aren't feeling it naturally. The more you practice this, the more frequently you will be joyful and from that state, your life will always change for the better.