Depression is a disease. Nobody knows why anyone gets it. You can take a hundred people and expose them to the same stress and some will end up depressed and some won't. No way to know why. Here's what I would say, though -- if you're in a relationship that makes you unhappy and you don't end it, that's a sign of something being wrong. Now, just because someone "claims" the other person is narcissistic doesn't mean that's true -- narcissism is also a mental illness and also has to be diagnosed to know if it's true. You only have one side of the story. But that side is from someone who appears to be unhappy with the relationship but is staying in it. There might be good reasons for that, such as a child or financial worry. You don't say. But whether it's depression or not or just insecurity or sadness or unhappiness is something that has to be diagnosed by a psychologist.