Being disappointed with a purchase does not make the seller responsible for your whole bad day.

A faulty item deserves a remedy; it does not deserve everything else you are carrying. When a bad week meets a broken thing, the complaint arrives at a volume the situation cannot account for, and the person in front of you absorbs a frustration that mostly came from somewhere else. Proportion is what separates a fair complaint from an unpleasant scene; it also gets you a better outcome, since people meet reasonable requests more willingly.

Before complaining, check whether the size of your feeling matches the size of the fault. Ask for the fix, and let the rest of the day be dealt with elsewhere.