A: Relationships 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.
B: Maybe they are that person when they’re dating, but then they change.
A: 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.