I find that medical doctors and engineers both commonly suffer from huge superiority complexes and/or narcissistic tendencies (and I say that as someone with an engineering degree). I think a huge part of it is that everyone is constantly fawning over them and telling them how smart they must be to have made it through the schooling that they begin to believe it (combined with the fact that many people conflate academic success with intelligence).
As a chemistry major, I went to undergrad with tons of pre-meds that went on to become medical doctors and then I went to grad school in an engineering department. Believe me, plenty of idiots hold a degree (even an advanced degree) in these areas. Most medical doctors are just mechanics that specialize in troubleshooting one particularly poorly designed device, and many engineers are convinced that because they know how to fix or optimize one problem, they can spend a few days/weeks to learn more than experts in unrelated fields.