Why does a spinning top slow down and wobble instead of staying perfectly upright?

A spinning top stays upright because angular momentum resists changes to its orientation. But friction at the contact point and drag from air gradually drain energy from the spin. As the rotation slows, gravity exerts a torque that makes the top precess, or wobble around a vertical axis. Eventually the spin is too weak to support the balance, and the top falls over. The same basic physics helps explain why bicycles are easier to balance while moving than standing still.