Here's why:
* Hair growth is determined by genetics and hormones. Gray hair happens because hair follicles stop producing melanin, the pigment that gives hair its color. Pulling out a hair doesn't change the underlying reason for graying.
* Pulling out a hair simply removes it from the follicle. The follicle remains, and a new hair will eventually grow in its place. However, this new hair will likely be the same color as the one you pulled out, meaning it will probably be gray.
So, while pulling out a gray hair might seem like a quick fix, it won't actually stop more gray hairs from appearing.