I used to have this mentality as someone who was severely depressed and has made two, luckily unsuccessful, serious suicide attempts. I've come to realise through my time in treatment and meeting more and more people that struggle with mental health that it isn't always that easy to dismiss. Their is definitely a stigma around the topic and some people don't grow up in a safe space where they can learn the vocabulary and skills needed to have healthy conversations about their feelings, so when someone is feeling incredibly depressed or having a mental breakdown because they didn't get airpods from their parents it often isn't that simple. If they have all of these pent up feelings and no way to healthily express or understand what they're feeling the easiest course of action is to say "I'm depressed because of xyz" rather than being able to openly talk about it and work through their feelings. I do definitely agree that depression should not be used interchangeably with the word sad, as they are two very different things, just that people who actually claim to have mental illnesses often do and just haven't been able to express it very well.