They can’t be themselves
When their whole personality and behavior changes when their SO is around.
It means that they can’t be themselves while being together and results in an unbalanced couple where one (or both) end up unhappy in the long run.
It gets tiresome to act like someone you’re not. –PsyQoWim
When they can’t make each other laugh. –FraggleOnFire
The next one is really important.