Threads are removed when they are in clear violation of the community guidelines (such as spam) and offer little in the way of legitimate support (keeping in mind always that we are first and foremost a self-help support community).

Threads are closed when we believe a part of the thread violates the community guidelines (usually due to one or more member reports), but we believe the thread may still offer some guidance or support to the OP. Sometimes we just can't pare down all the unsupportive replies to re-open the thread because there are simply too many of them. Sometimes replies take the thread on an unrelated tangent (thread hijacking), which may also result in the closure of the thread.

The answer to, "Why was my thread closed/removed?" is almost always because it violated a community guideline. We endeavor to notify the OP about the reason for a thread's closure via PM. We do not notify every member who posted to such a thread. As a team of volunteers, however, we're not perfect and sometimes we may close a thread and forget to notify the OP.

Hope this helps answer the question.