We lock threads for a number of reasons
1. If there are too many personal attacks
2. If the thread continually goes off topic, even after warnings from a mod/admin
3. If the topic has reached the end of it's discussion (for example, the thread "Here's hoping for a pairing of Pippa and Harry" was locked because Pippa has been proved to be with her boyfriend, Alex Loudon, and Harry has stated that he is 100% single)
4. Sometimes threads are too long, so they are locked, but then a new thread is opened up so the conversation can continue.