Generally speaking, what are some reasons that people ignore or do not follow their doctors’ or dentists’ advice?

Lazy
Money
Do not like healthy food

Incompetent.  I saw a doctor because my ankle had been slightly swollen and sore. He prescribed pills that would make me pee more so would reduce the swelling. I didn't think that seemed right but I took 'em for a month then returned,  swelling had not gone down so he gave me another prescription same pill double strength.  I did not take any and when I returned swelling had gone down on its own and my ankle healed.
"Those pills really work,  don't they?" he remarked.  Yeah sure, doc.

Sometimes, a doctor will say certain tests are warranted, but you don't think they are.  When I was really sick back in 2015 and no one seemed to be able to diagnose the problem, I had already had six MRIs, none of which showed anything.  When my doctor wanted me to do another one, I refused.  Same thing with the myriad repetitive blood tests.  Eighteen months later, the same way all the symptoms appeared, they all left and I've been OK since.

* The doctor hasn’t even reached puberty yet (Doogie Howser M.D.) 

* You watched them Google every question you asked (and it always took them to Facebook)

 * You’re sitting in the exam room naked except for a tiny smock that barley covers your left shoulder when the doctor comes in, and says “good morning Mrs. Jones”  (That's MR. Jones … get back to me once you’ve passed Anatomy 101)

 * What they told you doesn’t agree with WebMD (who’re they trying to kid?)

cause they dont want to