There's endogenous depression, reactive depression and there's low mood.
Almost everyone gets a low mood now and then, and cancer - plus cancer treatment - are cause enough.
Reactive depression - the result of, say, a loved one dying - can actually be a reaction to almost any stress, and individual experience varies, depending on past experiences, how you saw you parents coping, etc., etc. It is a clinical condition, it's more than low mood, and it's treatable, but often (not invariably) time limited anyway.
Endogenous depression - depression from within - can have all the above as 'triggers', but the cause is much more complex and generally unrelated to events. there is a genetic side to it; if it's in the family, you are more likely (but not certain) to get it. It also can link into to the hormone system and other apparently unrelated bodily functions. It's treatable, with varying success depending on a myriad factors (including taking the tablets!).
All in all, the proportion of people with cancer having depression is considerably higher than the general population - and HT alone can have a depressing influence.
Again, every GP should take it seriously, and if yours doesn't, you should change GPs and tell her why. There are some great GPs out there (I have one!), but, like most occupations, there some idiots. Best avoided.