Why do flavors need time to meld?

I make green smoothies for every day of the week. I hardly ever follow recipes and I always add spices and herbs some fresh some not.

I always knew chili’s and tomato based products are usually best the next day. There are a few other examples. What I don’t know is, why?

And how, when throwing together impromptu recipes combinations one can adjust to such changes. Personally I have gotten better through experimentation. Though other then saying flavors come through generally louder upon sitting and it is ok to have contrasting flavors at times to intensify and add complexity to a specific flavor.

Yet still often enough something I loved when I taste tested it upon making I had to choke down a day or two later.

Are there any guidlines or rules that can be applied in these instances?