Joy

Being happy comes from bitul.
When you think you deserve everything, you'll never have enough. On the other hand if the world doesn't only spin your way you'll be happier with what you've got.
Same thing spiritually and emotionally.
If all you do is think about yourself than today your happy 'cause you had a good day. Tomorrow you woke up on the wrong side so you're miserable. You don't care about what your doing, or Hashem for that matter, you care about yourself. So when you feel accomplished you're happy 'cause you're a somebody. When you're not accomplishing you are a nobody so you're miserable. Again you don't really care about anything other than yourself.
If you did you wouldn't be miserable, you'd work on the problem.
If I may talk about myself...
This past Sunday I related to this concept. there was a huge Israel 60 event in the JCC. We went to put on teffillin with people. Generally I'm pretty timid in such situations. For various reasons this time i gave some thought to the fact that I'm doing the Rebbe's work not mine. It's really not about me. And truth to tell I was on fire. Walking over to many a person with: "When was the last time you put on Teffillin?". B"H we had much success. In all each of us probably had about 25 Teffillin and in all over a hundred.
One older man walks by. When was the last time you put on Teffilin? Never. He was probably in his eighties. His wife had recently past away and he's just there to walk. I ask a few times. My uncle asks. Unfortunately he didn't put Teffillin on. I didn't feel like a failure. I said a kapitel Tehillim for him. And kept my eye out for him. He never came back.