One refers to a single person or thing, as well as the number one. If you’re looking for a whole number between zero and two, you can only find one.
One is all alone, but it at least it gets to be the first in line. It also gets to be the one and only, far and away the best, and one of a kind. If something (or someone) is "the one" you’ve been looking for, it’s exactly what you want. Sometimes one actually refers to more than one person being united in some way; if you and your friend are of "one mind," you think alike about something.