For as many different reasons and situations as there are marriages.

Mine ended because of abuse.

People change!

Are you begging the question here? Are you suggesting with your question that most marriages fail and asking why that is. Or are you asking just of those marriages that do fail, what the is most common reason.

If the former I'd suggest that on the statistics I'm looking at we're not quite there yet though the trend is certainly in that direction.

On the second matter sex, money communication and family.
[independent.co.uk]

Met a couple a while back who had been advised by their marriage counsellor to divorce. Apparently the main thing a marriage counsellor does is help a couple to communicate. But told this couple they were communicating really well - just wanted irreconcilable things from their lives.

No longer relevant in our society.

Mine recently ended after 35 years. It ended 3 years after the suicide of our son which I believe was the major factor.

These days, domestic violence plays a larger role in broken marriages. Marriage based solely on religious belief are also weakened by blaming an unknown or unknowable deity for problems instead of taking personal responsibility for the lapse in keeping the social contract. Personal intolerance is also big factor. As we age, we change. Failing to accept this change as it affects our relationship to each other and making healthy adjustments accordingly dooms the relationship to never ending dissatisfaction and estrangement. Oddly enough, some elderly persons gain better financial benefits from government programs if they divorce even if they continue their relationship after formally ending their contract with each other.

I don't think any marriage fails. The relationship just reached it's conclusion. Every relationship is going to end one way or another at some point.

For me, one word - drift.

Drifting apart. We were 24 when we got married, and then over the following 24 years became different people. Don't get me wrong - we tried. Christ in a sidecar, we tried to keep it working.

But we were different people at the end of it. At least it was amicable - no kids, no pets. And strangely we're better friends than we were for a while.

So, that's my experience - drift kills relationships. Gotta watch that shit was my lesson learned.

Every story is a different book

I only know why mine failed.
First one, both too young; I grew up/apart rapidly when my kids were born...he just wasn’t ready.
Second(the longest)...similar but years later. Grew in vastly different directions; no goals in common.
Third, there was abuse so I got out.
My conclusion is that if there’s not enough communication and compromise, it won’t work, no matter how much love is there.

I can't speak for "most" marriages failing. Personally I married too young, before I knew what to look for in a mate. I know I made many mistakes in my marriage. But I also feel like I tried my hardest to make it work. Probably longer than I should have. Unless BOTH parties are willing to put in effort to fix what's broken, I see no reason to stay and be unhappy. As atheist/agnostics we know we only get this one life. Why be unhappy?

Most marriages....l think maybe 53% fail.
I'd say, from experience, we marry the wrong people for the wrong reasons.

We are way more in touch with possibilities

I think it boils down to individuals thinking more about themselves instead of thinking like partners

If you figure that one out, you could make a lot of money.

Institutionalized inequalities that makes it acceptable to treat each other unfairly or disrespectfully, especially after there are children. Struggles with work-life balance can also add stresses to marriages.

[workingmother.com]

May be it is for living in the era of disposable things , ( silverware, bottles, lighters, razors, etc..) "The disposable mate"