A bright day can still contain a private sorrow that nobody outside can see.

Weather has nothing to do with what people are carrying. Somebody walking through sunshine may be in the middle of a diagnosis, a loss, or a fear they have told nobody about, and the brightness can make it stranger rather than easier. Public cheerfulness is not evidence of a good day. Most difficulty travels without any outward sign.

If you are the one carrying something on a lovely day, you do not have to match the weather. And when you greet people, leave a little room for an honest answer, so anybody who wants to say something true has somewhere to put it.