Most people do not consider the mental changes in someone affected to be an illness. They see it as a kind of "sag" as a result of too much work, a broken relationship, etc. And from this follows the widespread assumption that the suddenly changed person can overcome his problem through effort, perseverance, through "forcing yourself to do something".
Depression is an illness! Put simply, this illness is caused by a metabolic disorder in the brain. A person with a broken leg cannot suddenly walk again by "pulling himself together a bit more", which is often expected from those around depression patients.
Anyone who has never experienced depression firsthand cannot usually imagine how someone affected feels without comprehensive information. I'll let you understand depression from a sufferer's perspective. This is the basis for being able to help properly and effectively.