The word alone comes from a combination of the Middle English words "all" and "one," which is really all you need to know to understand the word. A person who is alone is all-in-one; there's no one else around.
Depending how you look at it, the word alone can have positive or negative connotations. A person with a bad attitude that no one wants to be around might find himself alone much of the time. But someone seeking solitude, might choose to be alone. Then there are those people or things that are so unique or special that they alone possess a certain quality. Superman alone could save the world from the evil hand of Lex Luthor.