Bob, threads get closed:
1) if they're items for sale. Communications between prospective buyers and the seller should be handled via PM, email, or phone.
2) if they don't comply with forum rules. An example of this might be someone who goes beyond venting and is just bashing. Personal attacks might be another reason.
3) if it's a duplicate thread - one will be deleted (as opposed to just closing it)
4) if there's been no activity (I think for a year) they age out and are closed.
I think that's it, but it's possible I might have missed other possible reasons.