Pride is one of the worst of the seven deadly sins mentioned in the Bible. It has been responsible for the downfall of various mythological figures like Satan in Paradise Lost, Faustus in Dr. Faustus by Christopher Marlowe and the like. However, pride can serve a positive and a productive purpose too.
Self-assurance is fine up to a point where a person's reach exceeds his grasp. When you begin to believe you are a better person than you actually are, or that you can do more than you are actually capable of, then self-assurance becomes a liability. Hence, one must believe in oneself but not be extremely haughty and overconfident.