Life, realities and hard choices

I just got off the phone with a long time friend of my. I am so suprised that this fellow is desperately looking for true love, but I thought he is in a marriage, yes, he still is, I thought he is happily married, probably not anymore. So, how does a marriage move from being sweet wedlock to being a frustrating entrapment with unhappy people forcing each other to stay. They have kids, they tangible monthly income, but they are no longer happy. My friend is getting depressed, I wish I could personally help him, but I believe he will greatly benefit from diverse views, hence this blog. Comments are eagerly anticipated