[jerojero]

There could be a lot of factors involved in this.

Retirement might be, in a way, a clear point in life when you get to actually think about your own mortality and life coming to an end in the next few decades. That alone could have a major impact in people developing depression at retirement.

What the poster says might still be true, people are being defined by their work and when that's over there's a huge gap they don't know how to fill. This might very well be a criticism on the role of work in society.

Ultimately though, you're right, there's no need to insult people when thinking critically about this issue.