I think that there is something to be said in terms of there being "needs" to be reach happiness. My happiness and that of the people I love is extremely important to me, and I definitely need more than food, water, shelter, and sleep (I can go without TV) to do it, especially as a depressive.
So, those things that I need, I pursue and make known to those who can happily provide. Things that I want, now, fall into different levels of course. I think with the chocolate and diet example, it simply comes down to what you want MORE. In that moment, do you want chocolate less than, more than, or as much as you want to lose weight? The answer to that question will determine the decision you make IN THAT MOMENT. You might regret it later, if upon reflection your level of want shifts, but we tend to make decisions in the moment in some situations (like, I'm at the store NOW, won't be here later, so I must choose NOW).
I tend to toss out things that I want as ideas, knowing that while I'm happy without having them met, I would at least temporarily be happier if they were. Needs, those are different. I need them to be happy.