In gay or straight relationships How do you fall out of love?

My grandparents were married for over 60 years. My parents have been married for almost 50 years! How do they do it? We all think 1 gay year in a relationship is equal to 3 straight years but come on give me a break on that. I just want to know how is it or how do people fall out of love? How does it happen? Maybe you were not in love to begin with? Tell me your opinion PLEASE!!