1. They become bored
2. They disappoint one another
3. They change or grow in different directions.
Well, one of those two people can become a binge drinking drug addict who endangers my son taking him to buy drugs from ex-cons who want sex from her in addition to money taken from our joint account while she's in prolonged unemployment. That's pretty much a red flag for me and we sort of drifted apart from there.
You learn things about the other person once you're living together and sometimes those things are impossible to live with.
I asked that question when my parents got divorced. They've been through hell and back together and randomly decided theyd had enough. I'll never understand it but the way they put it is that they've simply grown apart. A combination of issues that were never settled and arguments that were never resolved they said.
In my case... Too much ****, zero husband.