All the bacteria in our mouths have gotten there from the outside world.
When people share toothbrushes this opens up the possibility for cross-contamination of bacteria from one person to another. The number of bacteria on the toothbrush, the type of bacteria present (how aggressively infective they are), and the status of your own health and oral environment determine whether or not new bacteria can establish themselves on your gums or teeth from the toothbrush. Toothbrushes can also spread bugs that cause sore throats and other infections so watch out.
However, if you happen to share a toothbrush once or twice with a spouse or significant other with whom you are already swapping spit, you probably already have their cooties (all the more reason to choose your significant others carefully).
Even if you might be able to get away with sharing a toothbrush, I still would get my own. The health status of your mouth is the accumulation of many different factors and you don’t really want to add in additional unnecessary risks. One look at the gunk that accumulates on toothbrush bristles under a microscope is probably enough to make my point.