"Everything happens for a reason." It is a classic expression, but people understand it only in one particular frame. People believe that everything happens for a beneficial reason. People believe that all difficulties lead to a positive result, that all clouds have a silver lining, perhaps that we actually live in the best of all possible worlds. This is an important aspect of theodicy. I do believe that everything happens for a reason, but I do not believe it in this "it's all good" frame.
I believe it in the most literal sense possible. I literally believe that everything happens for a reason. Or, in other words, nothing is truly random or spontaneous. All actions, thoughts, beliefs, etc. occur because of some set of stimuli that came together and yielded this particular response.
This goes hand-in-hand with my ideas of fate. We can know what happens next when we understand all of the things that are happening now. This process works in forward and backward directions.
The greatest power of the writer is to create worlds and people. But what makes that power effective is when the world governs itself. If you can understand all of the inner workings of your world and its inhabitants, then you never have to worry about what happens next; it will be readily obvious from the flow of previous and present events.
Everything happens for a reason. If you can find out what the reason is, you will know everything that happens. How can you turn that down?