The Last Lecture, as Pausch introduced it, was about "my childhood dreams and how I have achieved them." Based on that announced purpose for the lecture, the major conflict addressed in the speech was man versus himself - finding ways to overcome obstacles that people create themselves as they strive toward their goals.
One of the phrases he suggested was, "experience is what you get when you didn't get what you wanted." The phrase was meant to convey the message that people need to learn from the challenges and conflicts that they come up against, and use those lessons to find a way to overcome the problem.
Another illustration to make his point came when he explained,
the brick walls are there for a reason. The brick walls are not there to keep us out. The brick walls are there to give us a chance to show how badly we want something.
His point was that people limit themselves by allowing problems to defeat them - man versus himself.