When you are taking a degree or you are still in high school, we are always competing with classmates to see who get the higher marks. This causes behavior such as refusal to teach others less knowledgeable than you for fear they will win you, he/she lose, you win attitude. However, I reckon that education is a non zero sum game, that is it is possible that both are well off from exchange of knowledge.
For example, I have a malay coursemate called Suhadah, I taught her programming in C++, and in return, she gave me her digital logic exam papers to read. When I teach her programming, I did not loose anything, instead I gained a deeper understanding of programming (a form of deep practice), and she gained the knowledge, in return, there is extra benefit of her sharing stuff with me! This will never happen if we never started sharing.
(Edit 22/March/2016: However, copying assignment is a zero sum game. You also made everyone unhappy, you help others to plagiarize, lecturer not happy, the person copying can’t learn.)