I agree with TWF that a lot of times it keeps going because the OP replies and engages with posters. And some threads gain traction because they touch on a common fear or concern (will my therapist end therapy) or the OP has been sharing an ongoing struggle and people are invested in how it gets resolved.

I also agree with growly that it's hard to know how to respond or what the OP is looking for. Will they be offended by the wrong comment? Do they want honest feedback that is challenging but potentially helpful, or just some assurance?