From "House M.D.: Clueless (#2.15)"
Marriages don't fail because couples get bored. They fail because, while they're dating, people pretend to be the person they think their partner wants and then - well, there's only so long you can keep that up.
Dr. Allison Cameron
Maybe they are that person when they're dating, but then they change.
People thinking their partner will change? That's another reason marriages fail. People don't change. At least not in any way that really matters.