What's wrong with this picture? If anything?

Every relationship has its drama. Couples get upset with eachother, they take things personal, jealousy can strike, etc..

But... what if your partner doesn't do any of those things? No matter what the situation or what you do, nothing seems to bother them. They seem completely content in every aspect of the relationship without a care in the world.
For example, say you straight up ignore them for a couple days and they're not even fazed by it. You start talking to them again and they act like nothing happened. It's all kosher and they're happy to hear from you.

Would you think there was something going on behind the scenes? Cheating maybe? Or do you think someone could really be that cool-headed and care free?

Or could it be something else?
What's wrong with this picture? If anything?
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