Humility enables a person to put aside all self-interest and tolerate the hardships of life. No longer are a person's actions dictated by fear of punishment, but out of love and good for all. Conversely, self interest or selfishness would be the most dangerous vice to the monastic community. With self-interest comes disobedience and lack of concern for the community. When acting with self-interest, an individual no longer considers the community as a priority.
Selfishness leads to pride, self gratification, gluttony, etc.
These sins are capable of undermining the fabric of the communal life. Obedience to superiors is necessary for the survival of a monastery. This is only possible if a person has learned true humility. You've only given examples for unbelievers. Through his idea of heaven, Gregory can be seen as a softer Augustine.