Why does a newborn cry?

I don’t know a single mother who doesn’t wonder if her baby has enough milk. After all, really, how can you understand that a child has eaten something if there are no dimensional divisions on the chest or stomach. I really want to keep everything under control. If there were clear instructions for the child, we would all be happy to follow them.
Where does this stupid information come from that if a child cries, it means he needs to change the diaper, and if the diaper is empty, then there is only one option left - the baby is hungry. Why? Is it always because of these two reasons that you get upset? If you think about it this way, then the most upsetting moments for us are when someone close to us leaves, when we are left alone. You are holding a human baby in your arms; it is small, but its needs are the same as those of a big one.
-Mom, take me in your arms so that I know that you are with me and will not go anywhere!
-Why are you crying again? The diaper is dry, and I just fed you. I guess I don't have enough milk and you're hungry. Let me give you the mixture.
And so the mother pours the mixture into a bottle and gives it to the crying child. The stream is strong and he has to drink.
“Well, he finally ate enough and fell asleep peacefully,” my mother rejoices.
But in fact, he has eaten so much that the body has no choice but to “switch off” the child and direct all its strength to digesting this heavy food. It’s as if you overate on the New Year holidays and now you’re lying on the couch and can’t get up - you feel so bad.
There is no other way to determine 100% whether a child is getting enough to eat other than by tracking weight. The whole world has come to the same values - how much a child (of a particular age) should gain at a minimum per day, per week, per month. If the weight grows enough, then there is enough milk. The baby sucks exactly as much as he wants. He learns to eat when he is hungry, which will allow him to not “eat up” all his problems as an adult. Eating is not the only answer to all questions, don’t you think?