The overwhelming majority of priests are extremely happy in their vocations. Why? Because they are doing what the Lord intended for their lives. Most priests will cite administering the Sacraments, preaching the Word, and helping people and their families as great sources of satisfaction. Ultimately, the source of happiness for any child of God is his or her relationship with Jesus Christ and the priest is given the privilege of acting in the person of Christ at key moments in the life of the Church. Studies consistently show that priests are very happy in their ministry, in far higher percentages than those studied in virtually any other life work. One recent and exhaustive study of the priesthood was done by Msgr. Stephen Rossetti, who published his findings in the very readable Why Priests Are Happy: A Study of the Psychological and Spiritual Health of Priests.
It is important to keep in mind that there is a difference between aloneness and loneliness. In the life of a priest, moments of solitude or aloneness are required for prayer, reflection, homily preparation, and rest. Many priests experience aloneness without feeling lonely. Further, in the midst of his ministry, a priest interacts with hundreds of individuals a week, and many life-giving friendships are enjoyed.
Still, no vocation, even marriage, is immune to loneliness. Therefore, a priest must always be vigilant in maintaining healthy relationships with family, friends, brother priests, religious brothers and sisters and parishioners.