Becoming part of a couple means it is not just about "me first" anymore. This goes double and triple when couples choose to have children. Kids have no say about who their parents are and count on the adults in their lives to make sure they have what they need emotionally and physically. Marriage and family means being concerned about, and responsive to, the needs and feelings of those we're close to. Far too many people seem to want the best of both worlds. They want to do what they want when they want how they want with whoever they want, but they also want the middle-class life with spouse and kids and white picket fence. In my experience, you can't have it both ways.
So, the key is balance. Happy couples understand that its not about "I" vs "we" as an either/or. Its a both... and... where the "I" and the "we" are important. There is a "you," a "me," and an "us." The "us" is a whole bigger than the sum of its parts, but it is each person being who they are that makes the unique relationship that is "us."