Look around you and you will see that this is true. People who are entitled (deficient in gratitude) are usually the people that are the most miserable. Be grateful for what you have and the ROI in Happy points in instantaneous.
This doesn't mean that you cannot be ambitious. You can be as ambitious as you want - as long as you remember what you have and are already happy with that. It's good to have goals, but if we want to move forward too fast, we don't appreciate and take full advantage of what we have worked so hard to accomplish. This is what is referred to as the "rat race." Live don't run. Enjoy the journey. Move forward, but make use of your mind more than of your yearnings. Listen to Marcus, emperor of Rome, the man who was once the most powerful on the face of the earth:
“Don’t set your mind on things you don’t possess as if they were yours, but count the blessings you actually possess and think how much you would desire them if they weren’t already yours. But watch yourself, that you don’t value these things to the point of being troubled if you should lose them.” - Marcus Aurelius