During the first four years of my marriage, I was completely consumed by the marriage itself, wanting to be only with my husband and practically doting on him - not at his request. It came to me rather suddenly one day that I was deliberately stifling myself, and that I wasn't happy with it. I needed friends with my interests, beyond what's for dinner and what movie we want to watch. I needed to spend time with people who came from different places and who knew different things.
When I returned to school this year (well, last year, now!), I moved away part-time and lived with a new friend, a girl who was a grad student in my major. It was the absolute best thing I could have ever done. My absence from my family made me appreciate my time with them, and they didn't take advantage of me in the same way. I had people who cared about opera with whom I could talk about it, and I didn't annoy my husband by trying to make him interested in the minutae of Puccini and Verdi.
Some people can get by with having no friends outside of their relationship, but I wouldn't dream of doing it again.