People Will Use Your Emotions Against You.
As sad as it is to say, people, even now (Never mind in a zombie apocalypse), will use your emotions against you if they think it will give them an advantage. Suppose someone sees that a certain thing elicits anger, sadness, or despair in another person. In that case, they will exploit that to get the desired response, usually with the intent of embarrassing the person in question in some way or clouding their judgment so as to not see some other underhanded tactic being used or, in the worst-case scenarios, to cause the person in question emotional pain, perhaps even to the point of suicide. All of this is, again, before we even get into a zombie apocalypse.
In the apocalypse, those same people would likely do the same things, except, now, they’re not simply looking to exploit your emotions to embarrass you or to distract, but to glean information from you about what supplies you have or weaknesses in your base, or things to exploit when they attack you, like something which will cause you to rush into a situation rather than think about what you’re doing.
Even within your group, people may attempt to play with your emotions to undermine your position within the group, particularly if it’s an influential one.
The trick to all of this is learning to properly get a handle on those emotions. If you’re easily provoked or pushed towards despair, people in a zombie apocalypse will use that against you, so you need to learn how to keep your emotions in check or, if possible, mask them entirely so that enemies can’t know what you’re thinking, and don’t know how to react.