We all have bad days sometimes, I understand that. It's not the end of the world (hopefully) and if you focus on all the crap and lament your bad luck, things will only get worse. I get that. However, I find it silly that there are some people who are very clearly having a bad day and scream, wild-eyed, in response to everyone's well-meaning regards, "I'm just FINE!" as though that would convince even themselves.
What's wrong with embracing the fact that you woke up late, maybe your laptop really is trying to eat the data that took you three horrific hours to compile and you've broken off the heel to your shoe? It's a bad day - not the end of the world. When someone wishes you a better day, suck it up, say thank you and move on. We do mean well and we sympathise with the crud you're desperately trying to ignore.
You're not fooling anyone.
Now, an appropriate video for your viewing pleasure:
By the way, yes, I am having a good day today, thanks!