Why is it frowned upon to eat the last slice of pizza? I think there are different reasons.:
1. It may e perceived as selfishness which translates into lack of concern for others.
2. It can also be perceived as gluttony and even greed, both of which are considered deadly sins or capital vices in Christianity and which would explain why is frown upon in a cultural environment with a high Christian influence. However, these are not highly desirable qualities in many other cultures.