As you make progress toward a goal, it is not necessarily true that the next step will be similar to the previous steps. The last step can be unexpected. In the book In Pursuit of the PhD, William Bowen and Neil Rudenstine calculated that fewer than 65 percent of people who start PhD programs finish them. I think this result is partly because students don't understand the the nature of earning a PhD. The last step of the process is to contribute to knowledge, which is unlike the previous steps. Elementary school is like learning to ride a tricycle. High school is like learning to ride a bicycle. College is like learning to drive a car. A master's degree is like learning to drive a racecar. Students often think that the next step is more of the same, like learning to fly an airplane. On the contrary, the PhD is like learning to design a new car. Instead of taking in more knowledge, you have to create knowledge. You have to discover (and then share with others) something that nobody has ever known before.

Bowen, W. and N. Rudenstine. 1992. In Pursuit of the PhD. Princeton, N.J.: Princeton University Press.