Can you only be depressed if something is biologically wrong with your brain?

ive never truly believed in depression. yeah we can have moments of sadness due to stuff in our lives, but i just dont really see how feelings of all things can overpower someones lifestyle and just ruin their life. but my dad told me that its a biological thing, like the signals your nerves send to your brain or whatever and anyway, its medically proven, right?if so, if one claims to be depressed but isnt due to evidence like that, then theyre just being weak or fakers right? or do i have that wrong?