Regret or disappointment in any aspect is made up when Expectations do not meet Reality: Expectations=Reality:at least being satisfied. Remember, people are satisfied with working as waitresses or mowing lawns, etc.
Now the expectations of each people -even in the same speciality - are not the same. It depends on age, previous life experiences, their life as children, country of origin, what they expected when they got into medical school and residency.
EXAMPLE: Lots of doctors from before HMO's/Prior Auth/etc hate their current job. They were present, and miss, the days the doctors decided on what to spend, where to spend it, how much to charge - but those days are gone - now Insurance companies and Administration decide.
Others were forced into specialites they did not want to go into. Maybe someone wasn't a stellar student/test taker - wanted ortho or Plastics but ended up in General Surgery. These people are most likely bitter/angry resulting in regret and disappointment.
I am willing to bet that almost every Attending/resident can name positives and negatives about EVERY field.