I usually read the whole thread before I post (if I intend to post). And sometimes by the time I've finished reading the whole thread I'm too tired to post.
The biggest reason, though, is that I usually find it hard to organize my thoughts into a coherent message. Sometimes it feels as though my thoughts are too scattered, too big, or too complicated -- or my opinions/feelings too strong -- to express coherently and succinctly. It's quite frustrating. For that reason I often skip out on responding to a lot of threads that I actually find very interesting and have strong feelings about and end up posting in lighter, "fluffier" topics where I can answer by posting just a couple of sentences or a few words.
What's strange is that I usually don't have the same problem expressing myself in writing when I'm writing for myself.