My main motivation is extreme phobia of flying, although to be honest, I am also pretty nervous aboard the train at times when the track gets rough. But after several LD trips, including Coast Starlight, CA Zephyr, and Empire Builder, I now love the journey itself. You sit back and relax while gazing at scenery that can be truly spectacular. Some of the LD trains go through scenery that you just can't see from a car or bus, and of course there is really no scenery on a plane.
However, I do always caution people considering a train trip that they need to expect delays and that it is insanely expensive to book a sleeper. I know a lot of people who would never consider the train, because they only care about arriving at their destination as soon as possible. But I know others who are very intrigued by the idea of the journey itself being part of the experience, and there is a certain romance to a LD train trip that is certainly not part of an airplane trip.