But they can be depressed. The economy can be depressed. We have depressions in the weatjer. Unless they are claiming clinical depression without diagnosis yheir usage is not proplatic. Correcting them is what should produce the eye roll.
Let me try to explain this in a less touchy manner: If I said, "I can literally feed a small village!" when I meant figuratively instead of literally, people would think I was silly or completely ignorant of the basic definition of that word (yes, in slang it can be used that way, but that doesn't make it correct). Economic and weather depressions allude to the mental illness, but they are not a person saying "I am depressed because of [insert trivial thing]." That's ridiculous and untrue.
Unless you can hear what thoughts are going through a persons mind, there is no way to determine if their use of "I'm depressed" over a soda is hyperbole or not. Getting angry at them for saying it and trying to convince them that what they are feeling doesn't meet your criteria for depressed is just as bad as those that armchair diagnose mental illnesses in others. Depression isn't like a light switch -- all on or all off. There are many degrees of depression including milder forms that might not be as sever as "I can't get out of bed". That doesn't make them less valid. Depression really doesn't need a trigger event, it can just be a general feeling of sadness without a root cause, but that's difficult for people to accept so it's easier to assign a cause to whatever is in front of them, including something as insignificant as they were looking forward to a treat, and now they can't have it.
I'm aware that there are different levels of depression, and that there is no root cause. I'm not always that depressed, but I can get that way. Feeling upset is almost the opposite of what I experience. Sure, I feel horrible, but mostly it's just numbness. I express emotion because I have to, not because I can, so to use the term to mean sad or upset boggles my mind.
It's also an incredibly interesting assumption that I go around being the PC police. I talk about these things with my family and people whom I trust; I don't go barging at people, chiding them for their incorrect usage. That would be horribly, horribly rude, and I would be the one worthy of chastisement. Just because I speak my mind to some extent in this forum doesn't mean that I lack a brain to mouth filter when I interact with people in person.