Yet another case, many times we will respond (or not) to a questions with another question, to elicit deeper considerations. In my experience, GPTs will answer any question without thought, pun intended :]
The original question was, given that there were X comments which could be replied to on this page, how and why did the poster decide to respond to the one to which they responded, rather than either "all" (like ChatGPT would) or "none?"
If you mean feeding it the messages one at a time, then, sure, it will respond to each of them because that's what its default "goal function" is. (I assume that we're talking about a single "conversation" here so the context is retained – feeding it each comment out-of-context would be very unfair.)
If you prompt it to only reply to messages it considers "interesting" in some way, and return "I'll skip this" for the others, it would certainly do that.
How does it determine how to respond to any prompt? Exactly. It might reasonably predict responses to those comments that, according to its black box, are somehow most likely to receive a reply. Different GPT instances prompted or trained to have different personalities would choose differently based on their priors and biases.
I'm pretty sure the vast majority of human beings is able to reason without really having much idea at all as to how exactly they do it. Educated humans can use language to talk about reasoning in terms of logic, and even make logic engines out of doped silicon and whatnot, but as of now, none of us has any idea of how reasoning is implemented in the mass of fat and protein that's inside our heads.