Usually this is NOT a negative card. The Fool represents the chaos from which everything began. This card technically doesn't have a number (since its "number" is zero), so the Fool isn't really in sequence with the other cards; he's an "outsider."
Arthur Edward Waite, when sequencing the Trumps, put the Fool between Judgement (#20) and The World (#21).
The Fool can represent a time of newness, when some aspect of the one's life has been "re-started" - his career, his romantic pursuits, his intellectual pursuits. The Fool is "back at zero" and may feel remarkably free, lighthearted, and refreshed, as if he were being given a second chance.
He is the embodiment of the Oriental concept of mu shin, the state of "no-mind" which is achieved at the higher levels of martial arts training.
Sometimes you must seem to be a fool and ask a question in order to gain wisdom.
In medieval courts, the jester/fool was not expected to follow the same rules as others. He could observe and then "poke fun."
The Fool was the original "wild card." Some writers say that he is the ancestor of the Joker, and is therefore the only trump card that ended up in the modern poker deck.