He is not some sort of ideally charismatic person whose primary quality is that he’s easy to get along with. Rather, he’s a demanding CEO with a monomaniacal focus on making Facebook succeed in its mission. This is not to say that he’s mean – he’s a perfectly nice guy on a personal level; it’s just that professionally, he is focused on getting it done, and has a limited tolerance for emotional fragility in the people he needs to help him execute on that mission.