How many of those lifetimes did the captain and their engineer look at each other and think "I wish you were mine"? The captain isn't always the captain in those other worlds, sometimes they're just them, and Mark is not always their head engineer. How many times did the other Marks see the captain and wonder why they weren't the same person they remembered? How many of those lifetimes did they see each other and feel like things were just slightly off, like they were blind and someone shifted the furniture in their house by like, an inch. And how many times did they see different versions of each other, wish they were still just The Captain and The Head Engineer, and still love each other anyway?