It's a tough question, because I feel like it's the lynchpin that all other moral questions rely on. You can't say "should" until you know why - if you think "X should happen", or "Y should be allowed", and you can't back that up by explaining what goals that encourages and what side effects it will have, then you need to sit down and untangle your thoughts a bit more.
I've had ideas in the past, but I've never really managed to boil everything down to something that I felt was correct.
I think I may have managed it, finally.
The purpose of the universe is to maximize the variety of joy experienced.
I can't find a hole in this, and I can't come up with anything further underlying it.
All of it's important, note. You can't just maximize joy without variety, because then you run into the problem where everyone is doing the same thing forever and there is no color or flavor to it. The word "experienced" is vital, because "experienced" implies sentience to experience it.
It's the first time I've come up with a purpose of the universe that I believe can be teased into a full moral system, all the way down to laws and the purpose of capitalism. With, admittedly, some difficulty.
Anyway, there you have it. In a few years I'll have come up with something better.