The expectation confirmation theory[0].

Someone else’s unrealistic assessment frame expectations, especially when they are attempting to speak from a place of authority, which they were. When reality doesn’t meet or exceed those expectations it creates disappointment. The expectations they set were impossibly high.

This is a pretty common thing. I’m sure we’ve all been disappointed by a movie or restaurant that a friend hyped up endlessly, which really didn’t live up to the expectations that were set. It’s the same deal here.

[0] https://en.wikipedia.org/wiki/Expectation_confirmation_theor...