Well, it was a small store, he was both the owner and manager and he went out of business so he probably was stressed about the store not doing well enough to stay in business and ended up snapping at you that day. You were young and you were in no position to understand "his" stress level which could have been overwhelming him as he may have been very worried about all the things he was responsible for that he was not able to keep up with.

When we are young we simply don't have the capacity to put ourselves in the shoes of someone else because we simply don't have enough life experience to have that ability. That man was not really thinking about you, he was clearly thinking about his own problems probably noticed a box that had not been broken up and vented about it as he was already in an angry mindset.