Depression is like a broken bone it's a physical problem with a person's body beyond their control they need help to fix it.
People who have open-heart surgery have a 40% chance of developing depression it's not that all their life they were dealing with depression this is a sudden change. That's terrifying to see depression develop so quickly like a virus suddenly changing a person almost overnight.
If a person impulsively and mortally harms themselves because they couldn't control their actions due to chemical imbalance in their brain (depression) that would certainly be upsetting.
Knowing a person's actions were out of their control and they could have been helped or healed that's quite sad. We need to help people not accept it or mock it since it is just as much a medical problem as a broken bone, and it can be treated.
There are cases of depression that result from health issues, but these definitely aren't the majority. This is just a hypothesis society has put forth to avoid taking a long hard look at itself.
Selfish is inherently a negative word, though. Maybe self focused or self motivated is still accurate and less negative.
I have people in my life who have been struggling with mental illness for many years. If any of them decide that they no longer have the will to fight, I would not think less of them (or call them selfish) if they decide to end their own life. If I did, I would be selfish.