The reason why is because when people post in your thread, you get .1 of a star. Therefore, they are doing you a favor for posting in your thread.
Maybe some people just start a thread so they can gain stars, but won't actually post that much in them since if you post in your own thread you don't get any stars.

Therefore! I feel it is a nice way of thanking people who post in your thread, to give stuff to them since they are giving to you.

YAY