The two major reasons for doing absolutely anything in the world: happiness and duty.
If something gives you happiness, even if slight, then it’s probably worth doing. You’re with someone (hopefully) because they make you happy. You have a job because doing it (or at least its results) make you happier. You take trips because you get to experience happy moments, either by yourself or with other people.
Duty is the second only in name. You take on a mission because it’s the right thing to do.
If something gives you happiness, in general, you do it. If something is your duty, you also do it. If it does neither, it’s safe to say that you can probably avoid it.
There are two major conflicts here, though. The first is making the distinction between short term happiness and long term. Sometimes something makes you miserable in the moment but feels great later. My books are generally like this. I work through them but they’re rarely enjoyable in the moment – in comparison to, say, blog posts, which are generally really fun to write as I do them. Sometimes, you have to sacrifice short-term happiness for a potential in long-term happiness through either achievement, or satisfaction, or whatever else. Here the trick is to know when to make that decision.
The second problem occurs when happiness conflicts with duty. Then you have a decision to make, and in my opinion, these are among the most difficult and straining situations you can experience.
The best situation, of course, is when duty and happiness come together into purpose. Then, you know you’ve really got something.