One of the reasons animal protein is so desirable is that it satiates one of our biggest anxieties, which is an empty belly. That doesn't mean the stomach knows exactly how to signal the brain and say, wait a second, you could satiate me with cantaloupe or lettuce. The stomach just knows it's full. It's interesting how we quench thirst by drinking water, but it's actually the water hitting the back of the throat that delivers the comfort.
The body communicates with itself in fascinating ways. We just aren't always intellectually equipped to interpret the data and make different choices. So we have to learn. If your goal is the optimal human diet, the first step is to eliminate processed food. That journey will take you many years, and it will require psychological and philosophical adjustments. It always does.