Of commitments

Just found out today that someone close to me broke off with his girlfriend after a long 6 year relation. It saddens me when I hear something like this. Not that I want them to stay together and be unhappy, but makes me wonder at what and how things can go sour in a relation that makes one take a decision like that. I have been married for 6 years now and I cannot imagine any other life than what I have now. We had known each other for 4 years before we got married and we had our share of fights and misunderstandings before we actually tied the knot. But we've hardly ever had a major disagreement or fight since. Are we the weird ones or is the world around us too complicated for us to understand?