Another possible explanation is that there's something about some people's personality that drives them to do both. Neither one is easy. Both require dedication

Another possible explanation is that there's something about some people's personality that drives them to do both. Neither one is easy. Both require dedication and hard work. The willingness to tough it out and do what it takes can apply to both. Along the same lines, it might be a personality trait where the person is prone to excess in certain ways. The article says the aim of a high-income is to live comfortably, but I think you can live much more comfortably putting in less work and getting a moderate income. Higher-income is something that happens when you want to go beyond comfortable. Similarly, you can stay in shape without running a marathon. A marathon (or other endurance activity) appeals to people who are looking for more than just the practical benefits.