Why does burned food (like toast, etc) taste bad?
We are programmed to enjoy indicators of nutrition, energy, and other necessary things that might be hard to come by.
Things like sugars, fats, salt.
Burnt things dont typically have those values, and so they lack the indicators: being sweet. Rich. Salty.
Rotten foods contain dangerous levels of bacteria and other chemicals that can harm us, so we’re programmed to avoid and dislike those signifiers (like smell, and when something tastes “off”.)
Caramelization. Sugar is obviously sweet, but it changes chemically when it burns. This creates all sorts of chemicals left over (did someone say carcinogens?) many of which are bitter.
When I was young my siblings and I used to love the burnt outside pieces from the roast beef at Sunday dinner. My father was old school and liked everything very well done.
Because your brain is wired to warn you when things are dangerous to eat. Taste is not an objective quality, but rather how our brains evaluate and associate what it experiences. We have evolved to dislike unripe fruit, rotting food, and other things to keep us from eating harmful things.
This misfires in some people, who end up craving non-food things like dirt, a condition called pica.