My reasons for posting in a thread vary. Sometimes I'll do it because the topic interests me, and sometimes I do it because I can't believe someone was idiotic enough to start a certain thread, prompting me to ask "seriously?"
I'll spout my thoughts or opinions regardless of which member started the thead, although there are a few here who's posts make me cringe when reading them. (I'm sure my posts have that same effect on a few of you as well)