Basically, the human body has an in-built limit to how many times it can be forced to move across the timeline. It is a hard limit; it can't be known, and it most certainly can't be changed. Its value is different in any given individual, and - although many have theorized - nobody knows whether it depends on genes, luck, physical health, spiritual inclination or pure happenstance. It happens to every Time Agent sooner or later... even you.