Why the fuck ask someone how they're doing if you're not gonna just let them feel what they feel. Does reminding someone that there are worse things in the world make them instantly think, "Oh shit, he's right, I should be grateful!" Sure, it brings some perspective, but now, not only are you feeling shitty for whatever reason, you feel shittier for not thinking about these other people who have it worse on top of the shitty you already felt.
If nothing else but this opinion gets burned into your mind, good. Don't fucking ask people how they're doing and invalidate it if it's not a pleasing answer. Just because you're uncomfortable with the response doesn't mean that person shouldn't feel what they feel.