Each one of us is called upon to serve God in many and various ways. Monasticism is a particular calling, a special way for man to serve God and mankind.
Every man has a “particular gift”, as St. Paul puts it. The calling for one to become a monk is just one such gift.
God himself speaks to the monk to be (offering signs and in the form of an inner voice), or through his Spiritual Leader (Father).
Monasticism is one of the ways for man to reach the Divine. To attain this state monks practice solitude, prayer, exercise, obedience to their Spiritual Father, service and fasting.