Some of the reasons to why a thread may be closed are when it starts to degenerate into personal attacks on other forum members/Frontier/ FD staff this behaviour is not acceptable in real life and I always fail to understand why some feel that it's fine to do so on the forum, a mod will at that point most likely step in and request that the personal attacks stop and that thread stays on topic. if it continues the thread may be temporarily or permanently closed. If a thread becomes circular with the same points being hashed over and over with no progression and logical conclusion it may be decided that the thread has run it's course, it's not a slight on the OP or anyone else just that everything that can be said has been.
The problem I have with the above is that often it's the defenders of FD/ED that form the basis of those reasons for closure. Be it their "okay, bye, don't let the door hit you on the way out", "don't like it, don't play it", "troll alert" type comments, their descent into "humour" (aka taking the thread off topic) and outright calls for the thread to be closed for some reason or other. It's happened a lot.
What I would do? Less thread posted warnings about "keep it civil", "keep on topic", etc and more actual warnings issued. The rules are there, IMHO the mods should use them instead of closing (or threatening to close) a thread when certain people are taking it off-topic, or in circles or whatever... because often, it's not the critical OP and ones who may agree with him/her who are guilty. Basically, punish the actual offender, not the thread itself.