First, most common mismatch: different goals. The things I'm trying to achieve are not the things someone else is trying to achieve, which makes cooperation difficult. Or we might be trying to achieve the same thing, effectively competing for the same resources.

Second, less common but still common mismatch: different emotional responses. If an event happens and I react in way A while the other person reacts in way B, either of us needs to suppress the emotional reaction, which is exhausting. I'm not willing to do that, unless genuineness is not expected by definition (at work, at family reunion).