It tends to be those posts where I'm most likely to want to add complete strangers to my Friends List, because I find people interesting when they make their points on whatever's being stated.
I find myself especially drawn to two categories of people, it seems: people fundamentally similar to me with similar backgrounds who drew different conclusions, and people very different from me who drew the same conclusion. I think in the former case, I'm interested to find someone with similar thought patterns using them to reach a very different conclusion, and equally interested to see a conclusion I understand, and a very different way of coming about it. I guess I find people then because those are the posts I pay most attention to- people like me who agree with me don't get my attention unless the comment thread gets long, and people who drew a different conclusion to me through very different thought processes I'm very likely to respectfully disagree with if I see something I'm interested in responding directly to, but I often can't find enough common ground to keep my attention.
I'm not sure why, really; those are just guesses. It's mostly an observation of my own behavior.