I went to the doctor and told him I had a phobia of tall buildings.

This joke gets its chuckle from a classic bit of wordplay, specifically a pun. The phrase "get over it" is the star here, doing double duty. On one hand, it's common advice for overcoming a problem or fear, meaning to simply move past it mentally. On the other, it literally suggests physically climbing *over* a tall building, which is precisely what someone with a phobia of tall buildings would dread. The humor comes from the doctor's seemingly unhelpful, yet perfectly literal, prescription for the patient's very specific fear.

The fear of tall buildings often falls under acrophobia, the fear of heights, which is a real and sometimes debilitating condition. People seek professional help from doctors or therapists to manage such phobias, hoping for strategies or treatments to lessen their anxiety. The joke playfully subverts this expectation, presenting the doctor not as a provider of complex solutions, but as someone offering a comically simplistic, and ironically challenging, piece of advice.

Ultimately, the joke's charm lies in its straightforward cleverness. It takes a common idiom and twists it into a literal command, highlighting the absurdity of the situation while delivering a quick, relatable punchline. It’s a gentle reminder that sometimes, the simplest advice can be the funniest, especially when it misses the mark in such a delightfully literal way.