That's a tough question. What I have found over the years is that most projects and commissions tend to be exciting in some way: either the subject matter or composition captures your imagination, or the client is one you consider important and you want to honor them, or the audience is one you particularly want to connect with. And it's important to find that exciting element and hold on to it, as that feeling will keep you working on the project with enthusiasm through the difficult stages (and there usually are some of these for every painting!)