Payment fatigue is an irrational thing, at least in the context of what a rational economic agent is. Rationally, you pay for what you value, at roughly what it is valued to you, and you don't pay for you want you don't value. Transactions are fatiguing precisely because we are not rational agents. Humans live primarily on emotion and ingrained behavior, with small bursts of applying logic.
In general, the "rational agent" abstraction is so flexible it can represent almost any real-world behaviour (which is why it's a bad model, not that humans are fundamentally irrational in some way).