It's a choice that you must make everyday. It sounds so easy, doesn't it? But the longer you are with someone, the more you realize the intricacies, the delicate nature of commitment. Some days, some weeks, maybe some years, you don't choose your partner, you just coast, passing by one another in the halls, going out to dinner, conducting the business of life.
My advice: take time to choose everyday. Make time. Your life will be so much more meaningful and your life more fulfilling.