From time to time, lobsters have to leave their shells in order to grow. They need the shell to protect them from being torn apart, yet when they grow, the old shell must be abandoned. If they did not abandon it, the old shell would soon become their prison–and finally their casket.
The tricky part for the lobster is the brief period of time between when the old shell is discarded and the new one is formed. During that terribly vulnerable period, the transition must be scary to the lobster. Currents gleefully cartwheel them from coral to kelp. Hungry schools of fish are ready to make them a part of their food chain. For awhile at least, that old shell must look pretty good.
We are not so different from lobsters. To change and grow, we must sometimes shed our shells–a structure, a framework–we've depended on. Discipleship means being so committed to Christ that when he bids us to follow, we will change, risk, grow, and leave our "shells" behind.