What's Marriage For, Anyway?

I was wondering, yesterday, what, for the most part, makes a couple decide to divorce. I mean, ruling out beatings, infidelities, late-discovered homosexuality and things like huffing addictions...

For your average couple that has been married for a long time, what is the trigger?

I know this sounds like a simplistic question, but nothing is simple.

If a couple have never been in love, I get that they might eventually "call it quits" -- when their life together gives them no comfort; no sense of union; no romance or sensual fire... A time comes, I suppose, when they have seen what some couples have between them, emotionally, and they don't and they decide to go looking for fulfillment.

One wonders, of course, why they got married in the first place if this feeling didn't exist, but I'm in no position to speculate about specifics. Could have been a lack of experience; not knowing when things were lame because of a lack of comparison. Could have been the result of falling into a routine with someone and then going the next step, to marriage, because it was expected. (My dad did always warn me about falling into a comfort zone and becoming blind to what was wrong in a relationship.)

Some "hang in there" until the kids are in college and then they drop the hatchet.