With regard to this question, Adam Smith is, without question, more correct than Karl Marx. Marx thought that people (ideally) work because they love working and they love what they do. He thought that capitalism perverted that relationship and made people lose their love of work. This may be true for some people, but most people work largely because they need the money.
It is certainly true that there are many professions where people love their work. However, it is likely that most people in most of those professions would not work as much if they did not have to. I know many teachers who would prefer to work half-time if they could afford it. It is the rare person who would work just as long and hard as they do now if they were not paid for their efforts. Therefore, Smith was right when he said that people generally work for self-interest.