Well... i think after somebody else posts in a thread started by you, the thread is not more only "your", because it haves the opinions/comments of another person too.
As i think nobody haves a "greater" opinion than anybody, if there is a thread with, lets say, 20 replies, then all the 20 persons that posted in that thread are "owners", in a certain way, of it. Or, lets say, have the same "weight" in that thread.
Thats why i think nobody (even the thread starter) should be able to close threads. Only if it were with agreement of everybody that posted, what would be difficult.