Good question with a ton of answers.
Some people forgot that when you marry someone it is for life. They do not realize it till the 1st yr, 7 yrs, 13th,and the 20th.. years of marriage. Then they begin to realize.. "good grief, We still have another 50 or 60 years to go!"
These can be times when you have changed or grown into a different person than you were when you married, but you or your spouse still want the original person.. Or people grow apart. They work so hard on their goals, they forget to work that hard on their relationships.
Some people smother their spouses.. It is flattering in the beginning to be adored and wanted, chased and a person being a little jealous but after a while it becomes tiresome.
Some times the goals we originally had are not really what they thought they would be or what the consequences to having these goals would mean. We end up feeling we did all of that work and we are still not happy.
With keeping up with all of the work that is necessary to maintain it.
(Be careful what you wish for it may come true)
Some people marry, because they do not want to be alone or afraid they will never find someone.
Some marry because it is what is expected because living together or having sex outside of marriage is considered a sin. Then they marry this forbidden fruit and realize, it was not a good match after all.
Some were not emotionally mature when they married.
Some were not being their honest selves when they married. (Oprah ~ Women can hold in their crazy for a long, long time.. even decades to get a man.)
Some men and women, just cannot be faithful.
Some people have issues and do not want to face them and work on themselves.
Some are just lazy and want and expect their spouses to do everything for them.
Some people complain but do not want to be part of the solutions.
Some people need to totally control other people.
Some people get tired of the responsibilities. The grass is greener..
Marriage takes a lot of effort.. Add, in laws, work, children, health..
it is tough if the 2 people involved cannot be honest and ask clearly and kindly for what they need. Respect, Appreciations and communication is the secret to the good marriages. There are no secrets, no assumptions and no taking for granted. You want the very best for your spouse, before you want it for yourself.