I don't think that it's necessarily a negative card. Yes, it can be, if you think of someone who is foolish and childish. However, to me it can also mean that he thinks of you as someone who is innocent, full of child-like enthusiasm, or who doesn't mind taking risks or embarking on new adventures. Think about it, being naïve has a different side -- that of someone who is willing to believe the best of people, who is still full of curiosity and sees the world as being full of possibilities. Being foolish can also be related to being the sort of person who is always willing to do things others would consider dangerous or outrageous.
Like many other cards, to me the Fool has both positive and negative qualities. What someone considers negative can be another person's positive and, more than that, many defects and qualities are closely linked and the only distinction is the context and the extent to which they affect people. For instance, being willing to see the best in everything can be a good thing, but it can also be bad in the eyes of an eternal pessimist, or if it leads you to get in trouble (or get others in trouble) very often.