What kind of person becomes a monk or a nun?
 
Most people have a variety of interests in their lives - family, career, hobbies, politics, religion etc. Of these interests one will be primary, usually family or career, while the others will be less important to them. When the study and practice of the Buddha's teachings becomes the most important thing in the person's life, when that takes precedence over all other things, then that person will probably be interested in becoming a monk or a nun.