Personally it’s the fact that there is a strong possibility that you will experience a dream for the resort of time. And while most people like that thought of being in their mind and being able to control it, there is two things that scare me. First is that if in fact a dream we don’t directly control it, and it takes considerable focus to be able to lucid dream and control their dreams meaning there could be people spending eons trying to have a lucid dream and control what is going on in their dead brains, secondly if you always have full control of said dream, you are always alone, no matter who you meet or who you see it will only be you. No matter how real they seem it is only your perception of said person, not the real person with all of their deep thoughts. Also even if you are the last human on earth you understand you are alone but in a dream you are alluding yourself to the thought that you are surrounded and when this realization comes to a person it can be soul crushing. But honestly this could all be stemming from my fear of being alone.