Why each is not entirely a contradiction but makes sense on some level.

Like the previous post, "bitter sweet" is an oxymoron. It comes from two Greek words, meaning sharp and dull. So, even from the word itself, you are able to get at the definition of the word.

Oxymorons are great to use in literature, because it makes the reader think. It is similar to a paradox in some ways. On the surface, it does not make sense at all, but with some more reflection, usually a great truth is unlocked with wit and cleverness.