"When all you have is a hammer, everything looks like a nail".
Imagine you walk into a doctor's office complaining of a rash on your arm.
The doctor has 15min to diagnose you, because there is a waiting room full of patients sitting outside.
And the doctor has seen similar rashes before, and in 75% of those cases, the rash was caused by the same bacterial infection.
She logically assumes that the same bacteria is causing your rash (it happened in 75% of the cases after all). She gives you an antibiotic cream and sends you on your way.
Except that your rash gets worse, and now it's causing serious pain. The antibiotic cream didn't work.
Before you blame the doctor, it's not entirely her fault. It worked for 75% of the cases after all! And she needs to see all these patients waiting patiently in the waiting area!
Doctors are human beings, and no matter how good or bad they are, they are still susceptible to human errors just like anyone else...
In fact, as this article states: doctors themselves admit to making diagnostic mistakes all the time!
This is why it's always a good idea to get multiple opinions.
How often do docs make diagnostic mistakes (from the article):
Emergency Room docs: 26%
General docs: 22%
Family docs: 18%
Internal Medicine docs: 15%
And those stats are in the US. I would expect worse stats in countries with less strict laws and regulations...
And when your doctor says: " Your problem is caused by X", ask him/her: "Ok, I understand. What else could cause it and what is the probability of this being something else?".