[jehb]

Why did you write this comment?

My intent in asking isn't just to be dismissive, but rather to point out that there are complex, overlapping reasons. It can be incredibly difficult to even pinpoint your own motivations, to say nothing of those of others.

A simplistic answer might be "because they want to have their thoughts read by others." Beyond that could be altruism, a desire for notoriety, wanting to change behavior, or many other intermingled goals. Most likely, the author didn't even consider "why" but rather, like all of us, just felt an intrinsic need to be heard. See also: social media.