I think money, fame, celebrity status, etc. often creates pressures on the relationship.

Kind of like how it sometimes pushes people into drug addictions. That's a stereotype that we've all heard about since kids, but it wasn't until relatively recently that I thought about it seriously that it occurred to me that a touring celebrity musician's life can be stressful and the drugs can be (or start out as) a pressure valve release. I think that part of it clicked to me when I heard a musician being interviewed about it, I forget which. Maybe it was a Beatle talking about their early amphetamine days.