IMO its a mix of...
a) being accountable for factors outside of your control i.e. I see my GCSE students for 5 hours a fortnight but I'm responsible (me, not them) if they don't achieve their target grade even if they are living in complete chaos, poor attendance, poor behaviour etc)
b) workload
c) behaviour
d) profession is being demonised by parents, press, public (we are all lazy, uncaring, bullys)
e) not having control in our own classrooms of how to teach, what to teach
f) constant large changes to curriculum/exam specs
g) being blamed for everything in society i.e. kids doing a, b, c it's the fault of the schools
h) being expected to be parents and not teachers - kids can't use cutlery it's upto teachers to teach them
Apologies for the lengthy list however for lots of trained teachers it could be one of the above, but usually its a mix of those listed