A couple's relationship can be both complicated and enhanced by outside relationships. Not all outside relationships will fit well into a couple's life together. Some of them, like those with mothers-in-law, can't exactly be avoided, but others can, and they should be kept separate from the couple's relationship. These friendships can enhance the couple's time together by giving each person time and space to nurture and develop interests that the partner doesn't necessarily share. A lunch date with a coworker is a much better outlet for discussing a work project than dinner at home. And sometimes it's really nice to be able to enjoy an inside joke without having to explain it to your significant other.
At the same time, it's healthy for couples to have friends they hang out with together. These friendships provide a way for a couple to be social together and explore common interests. They can also be a source of positive feedback for the relationship.
In the big picture, good relationships with family and friends, both singly and as a couple, can enhance the relationship and keep it lively and vital. Plus, research shows that people with a strong social network live longer, healthier lives.