Companions Can Cycle Out At Irregular Intervals
While Companions can come from various races, planets, and species, their life spans almost always are dwarfed by The Doctor. As such, these friends of Gallifrey’s rogue hero tend to come and go at irregularly paced intervals, leaving The Doctor on the search for new friends from time to time. Though, in some cases, they manage to make their way through an adventure without a stable Companion, taking someone local along for the ride. Much like the concept of Regenerations, the departure of Companions can sometimes be bittersweet, and other times downright devastating.