People differ in their willingness to share, as well as their reasons to do so. An open collaboration community of willing sharing members thrives on a virtuous cycle: increased sharing often offers stronger reasons for more people to share. However, it may also decline when the cycle goes the opposite direction and turns vicious. What determines the dividing line? We offer insights into this important question based on an analytic understanding of the concept of rational sharing, which is sharing for net gain in personal utility. In a nutshell, a community thriving on rational sharing is essentially an economic system, a platform for creating mutual benefit through exchanges.