Almost certainly you possess the remarkable capacity to put together explanations when you see something happening (in the street for instance).
A huddle of people are looking up at a tree, so there's probably a cat stuck up there. A man is sitting on a shop doorstep, and he's likely to be homeless and will ask you for money.
Of course we don't always get it right. Our assumptions can prove wrong. The tree observers could be council workers discussing a pruning job. The man in the doorway could be simply tying his shoe.
But there was no doubting the cause of the little scene I passed the other morning on the way to work. Outside the nearby children's nursery, a young Mum was smiling and waving through the window. I saw her first. A few paces on, I spotted her little boy inside, with the unhappiest face in the world.
He clearly didn't want to be left there.
As I walked on, it seemed mean and heartless of the mother to be smiling. Surely she'd be upset to see her son in such distress?
Thinking a bit more though, she was probably doing the right thing. Trying to get her little boy to see it as normal, nothing to get het up about.
And this is probably a good way to think about how you'd like others to be with you if and when your own mood is low. You hope they'll empathise with you. The last thing you'd want is for them to suddenly get as low as you.
It's a fine balance though, worth exploring when the boot's on the other foot and you're around someone else whose mood is low.
The answer is almost certainly to be yourself, and to behave as normally as possible.