Sometimes we fill the picture in favorably, but if there's anything in a comment that rubs us the wrong way—anything activating—then the picture we create is not a nice one. Since a forum like HN gets millions of posts a year, there are inevitably plenty of hooks to snag us in this way. If you stop and think about what that means experientially, it's sort of shocking: it means that we are surrounded by our own demons. Since everyone else is having the same experience, a forum like this is a community of people who feel surrounded by their own demons, not realizing that we are mostly creating them for ourselves. This is why disagreements degenerate so quickly, even though each party is sure that they're the one in good faith. It's also why you'll regularly see generalizations about how awful the community is, posted by users who participate in it every day. It's surprising that it can function at all.
By "demons" I just mean a composite of our own past painful experiences, which is the store we draw on when connecting the dots about someone else in a defensive way. This phenomenon isn't only online; it happens everywhere that people don't know each other well. But online, we know each other so little that there are more dots to connect.