Comment by quantadev

One intuitive explanation is that anything moving at the speed of light is experiencing no time at all (from the point of view of an observer) and if something is moving faster than light that means it's going backwards in time. (only massless virtual particles can)

If a clock stops or runs backwards that totally messes up "causality" which is about events interacting relative to a time order, and so time must exist for causality to make sense.