Why are some people lazier than others?

History gives us many vivid examples. For instance, Charles Darwin was incredibly lazy. His teachers and parents suffered a lot making him learnt grammar and math at school; often he fell asleep right at the middle of the lesson. He preferred fishing and shooting crows to doing sports and when he was a college student, he spent most of the time in pubs. Even when he got into science, he didn’t rush things and spent years writing his work.