Honestly, I just write what works best for the particular story, whether it is happy or sad. If I was partial to one or the other, the ending would feel contrived more often than not. My WIP in editing is a mixture of both and I think it works well--not a total downer, but the MC sure doesn't get what she wants. If I summed it up any other way it would be contrived, but as it is, it's satisfying. So I'm happy.