When Mickey Mouse first made an appearance, he rather quickly became a bit of a role model for children, to such an extent that Disney decided that they had to come up with a different character who would have a bit more of a negative edge to them. As Mickey was being held in such high regard, they believed that it would be wrong to bestow certain characteristics on him as that would just effectively confuse children.
As a result, Disney set to work on creating this alternative character, and it ultimately led to the appearance of Donald Duck. He was seen as being more negative in some of his traits as he was certainly far more short-tempered than Mickey could ever be. However, it turned out that he too would become extremely popular and his apparently negative traits would be the thing that would really draw people to him.