Why does some film actors and actresses usually changes their spouses?  

I don't see the point why do some actors and actresses usually change their spouses? Just like Angelina Jolie. She was divorced twice from actors Jonny Lee Miller and Billy Bob Thornton. And now, she is currently living with her domestic partner actor Brad Pitt. Their relationship attracted worldwide media attention. They have three adopted children and three biological children. In which I have no idea on how will they manage to grow them up and take care of them. Aside of having a good financial stability. I wonder if Jolie will change his partner Pitt::