I feel that part of other posters are right but to add: under normal circumstances we have two main brain systems, a fast system (Sys. 1) that is automatic and a slow system that is less reactive and filters through inputs (Sys. 2).
Emotions are guided by Sys. 1, but can be reigned in by Sys. 2. However, enough stress to the system, and repeated inputs will make Sys. 2 be less efficient, and Sys. 1 will do the driving while Sys. 2 takes a nap. That nap is just long enough to let impulses, such as lashing out at the world because angry, happen.
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