Why people get so angry in normal condition also?

Just went out to a shop for getting some print out. Since there was a formatting involved, I requested him to do so and agreed to pay him for the same.

He got annoyed and started firing me, saying what do I think of him. Is he working for money.
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I kept on saying sorry for few minutes, even though I didn't say anything offensive. He continued for 4/5 minutes and finally calmed down.

Out of curiosity, I tried to find out what was wrong with him and came to know that there is an issue in his family for which he was frustrated.

We must have control over our emotions, in every situation, and I am thinking to suggest him the book "The Four Agreements" to learn how to control the same - the way I started doing after reading that book.