Doctrine is a truth that comes to us through either divine revelation or natural law. Doctrine is something held to be a truth and thus must be believed. Doctrine cannot change since truth cannot change. Jesus cannot be God one day and then just a prophet the next. Whatever the Church teaches as doctrine is always true and must always be believed.
Discipline is something that the Church asks of us for spiritual enrichment or orderly running of affairs. For example, there is nothing inherently wrong with eating meat on a Friday. It is a discipline of the Church that on Fridays in Lent we abstain from eating meat for our spiritual enrichment. Disciplines, because they are not truths, can change. At one time the Church discipline was to abstain from eating meat on all Fridays of the year.
In the case of celibacy, there is nothing inherently wrong with married priests. In fact, the Eastern Catholic Churches has always had married priests. In the Western/Latin Church we have also had exceptions such as married Anglican clergy who converted to become Catholic priests and are married. The traditional discipline of celibacy has been asked of priests as an imitation of Jesus Christ. It spiritually connects the priest to his ministry in a way that is unique. A priest of the Latin Church is spiritually, physically, emotionally, etc. given completely over to his priestly ministry through the visible sign of celibacy.
