Please make that argument! I'd say disorder would refer to something unusual, whereas sense of self is very common among humans across different cultures. Am I misinformed?
It’s not a binary thing, to have or not have a sense of self. It’s the strength of that sensation that matters: can you let go of your own shortsighted selfishness to let something else grow? Unusually strong sense of self has been argued to be a cause of longterm depression. Comparison is the thief of joy after all.
You can grow past having a sense of self and still be depressed.
A closer overly simplified cause for many kinds of long term depression is pessimism and doubt. eg, "It will not work [so no need to consider trying]." It will not work is pessimism and not considering trying is doubt. Another terminology to describe this is learned helplessness.
But depression is far more complicated than just that. There is memory compartmentalization in a handful of types of depression, mostly episodic depression.
Sometimes it can be as subtle as not learning to note when you're having a good time. Someone who is depressed may, for example, have fun at an event, but the next time the same event pops up they may be hesitant to go again, because they didn't record when they were happy.