Both really... But saying thank you encourages people to respond again. To take your analogy, if I held the door open for you and you didnt say thanks or acknowledge it, then the next time you can open the door yourself!! It's the same in the forums.. If people don't feel appreciated they don't post, vice versa if they do feel needed they'll help even more. (I think)

Oh well, just my views anyway

cheers, shaun.