So your marriage failed. Why, when it was expected to last a lifetime?

In an attempt to get many answers, I have posted this question at different time slots so it attracts answers across the globe -- several times. I will pull the answers so far together and ask for some kind of refined next set ot thoughts: about cheating, changed life goals, interferring parents, loss of drive.... So help out, please, answer for yourself or others you know. Thank you.