Board Thread:General Discussion/@comment-8161155-20140516200942/@comment-15217257-20140517223909

I always thought that he had a normal name (Bill, Kyle, Eric) but kids called him "Little Dipper" because of his short size and birthmark. He liked the last part so he started calling himself "Dipper". This would also explain why he got so mad when Mabel was about to call him "Little Dipper". Your theory's make more sense then mine though...