Why do so many marriages fail?

Incompatibility
Cheating
Money
Kids

Cheating and mental illness as well as the stress of money issues. Also there's a lot of verbal and physical abuse that goes on towards the spouse and children in some cases.

Lies.

Drugs and Alcohol were the main causes of my failed marriages. Luckily for me, one divorced me when I was in Vietnam as she wasn't getting any money for her habits. Second one starting drinking heavily and just got mean. I couldn't take any more.

Too many people buy into the happily ever after myth. They assume that as long as you love each other and face everything as a couple you go through life with butterflies and rainbows around you. The truth is sometimes you are not going to even like your spouse very much. That doesn't mean you should split up. It means you try to understand the other person's point of view. There are some things my husband and I will never agree on. And sometimes he really ticks me off. There are times I have thought about walking out the door. But I know that if I let things cool down and we talk calmly and explain how and why we feel they we do we can work through it.

The most common reason is misunderstanding and other reason is cheating ,trust ,and anger

I've never been married so I am not sure what would cause so many to fall apart.

However statistically speaking, 100% of divorces are due to marriage.

From observation "Supermarket shopping." Ok, it may not be the cause but it's certainly a straw that could break the camels back. They should have marriage counsellors at the end of the till points.

Sadly those who are “lovers of themselves” are selfish and lack consideration for others. Husbands or wives who love only themselves are determined to get their own way. They are inflexible, unbending. Would such an attitude contribute to a happy marriage? No way! Love of money can also drive a wedge between a husband and a wife. This has came true in many marriages today. In their quest for wealth, many spouses ignore the needs of their mates, including the basic need for emotional support and regular, warm companionship.

Interesting question.

Just a few thoughts:

There are various causes, some proximal, some remote.

Alcohol and other drugs and spousal abuse may well indicate that a person lacks the capacity to be in a marriage in the first place.

After that, either a refusal to acknowledge---or an unawareness that--- the marriage vows you made bind your transcendental self (who you were, are and will be) to your partner for life.

Personally, I think growth really only happens through commitment. (As CS Lewis wrote, "Good, as it grows, differentiates itself not only from evil but also from other 'goods.'")

I maintain that two healthy adults are presumptively capable of having a successful marriage if they so choose.

Most people have no idea what they are getting into. No idea of what the word even means.

A lack of mutual trust, respect, and commitment.

Well here is what did mine in stress drug n alcohol abuse yes those were my problems my wife money money and dating a cop.  A big no no in my family

well my mom pretended to be nice tip she got my dad to marry her then she showed her true demon same with my friends parents.

There are many reasons marriages fail .. Too many to even count.  However, primarily it can be boiled down to two 'incompatability' and 'inability to compromise'.  It is sad how many people are mesmorized on the thought of getting married .. To the point where they even change their behaviour to attain it as a 'prize'.  They may act differently because they 'feel' differently. Sometimes, over time people drift apart because they simply don't have a solid enough foundation to hold them together.  People act differently when they are happy .. And if they eventually discover they are not happy, or their lives have taken a turn they are not happy with, their perspective of it changes. Things most often change as two people REALLY get to know each other over the course of time. 

Same reason relationships fail for dating. Sometimes you simply lose interest , or you are cheating/ lusting for someone else. They can also get into many fights etc.

Something that I didn't see mentioned in these excellent answers is the term, for lack of a better phrase ..."pre-existing conditions" , which are brought into all marriages.

Sometimes there's so much damage done by upbringing or other circumstances that it just can't be overcome ...and I agree with Tom Jackson in saying that that is a choice.

I have seen people actually realize problems they have from things their parents did,  or the way they raised them,  and made a choice to change-but I have also seen those that refuse to see they could possibly ever be in the wrong.

Who we are as we come into a marriage and who we decide to become in that marriage is really close to the top, if not at the top, of the list as to why marriages succeed or fail.