I think many doctors are arrogant, assume they're "more correct" than the patient. True, there may be hypochondriacs who think they have X disease, when it's something harmless, but there are doctors who dismiss serious concerns.
Once, I had what's known as a migraine with aura (or ocular migraine). No headache, but 20 minutes of weird, spinning lights. I looked it up, discovered what it was called, and told my doctor. He said, "We can't say it was a migraine, I get migraines, they're rough...were you using any illicit substances...were you sleep deprived?" The questions were fair, but the guy either didn't know what an ocular migraine was or didn't think a patient could know more than him.
...subsequent doctors, including an E-N-T and an optometrist, both said right away, "Yep, classic ocular migraine" when I described it to them. So, it wasn't I who was crazy, but some doctors think it's impossible for you to know something medically that they don't.
ALSO: I think I mentioned this. Recently, I floated the idea of Asperger's/Autism to my regular doctor. He said it's impossible for me to have it because I can make sentences, make eye contact, hold a job, etc. I told him that even if I don't have it, his general claims about Autism are patently false (this website if filled with diagnosed Aspies who did all those things and didn't get diagnosed until later in life). I'd rather the doctor simply admit he didn't know instead of acting like an expert on something that I knew more about.