In the early Church most priests were actually married. Over time, however, the understanding of the priesthood developed and many priests began to choose to live a celibate life 'for the sake of the kingdom' (Matthew 19:12). They fully give themselves to Christ and 'the affairs of the Lord' (1 Corinthians 7:32).
Just as in marriage spouses give themselves fully to each other ‘Ìâ‰âÂÌâ‰Ûù spirit and body ‘Ìâ‰âÂÌâ‰Ûù so a priest gives himself to the service of Christ for the Church. Imagine trying to give yourself fully to an entire parish and have a family to take care of! You'd have to make sure your family has food on the table and a roof over their heads and are spiritually and emotionally taken care of.
Then with the parish you have to make sure they are all spiritually fed, and all the sick are taken care of, all their spiritual direction, Confessions, Baptisms and Burials. That's not even mentioning all the administration stuff they have to take care of too!
Not that it's impossible to do, but it divides them and doesn't allow them to fully give themselves to either their parish or their family. In the end both the family and the parish would not have their father to the full extent. Instead priests make a full gift of self to the Church.
The priest stands in the place of Christ whose bride is the Church and so gives himself with an undivided heart to the Church. Since the Catholic Church is the priest's bride, they follow St. Paul's instructions to husbands 'to sanctify her, cleansing her by the bath of water with the word' and to 'love their wives as their own bodies' (Ephesians 5:26-28). He consecrates himself to the work of Christ, which is to prepare the Church for when Christ returns. Celibacy then is also a witness to the coming of the Kingdom of Heaven ‘Ìâ‰âÂÌâ‰Ûù where all the faithful will be united to Christ.
Priests choose to give up something so good and beautiful as marriage to be a visible reminder that there is something even greater waiting for us in heaven.