Differences exist at three levels

In terms of conditions

The time in which we live may represent a benefit for one and a disaster for the other. Automation technology creates new jobs for some and unemployment for others.

In terms of skills and experience 

Much can be learned and taught, but talent, age and the time in which we live set limits. We can’t, for example, practice more than a few professions at the same time (and keep ourselves up-to-date).

In terms of motivation and goals

People are motivated through actions or matters that bring them closer to their goals. However, they don't always know exactly what their own goals are. People often 'borrow' goals from others. It's about a 'borrowed' interpretation of what 'sharing fairly' means. That doesn’t always turn out well, as shown in the following example:

A brother and sister are arguing with each other about an orange. Their father intervenes and says, “It’s not difficult to settle the matter”. He takes a knife, cuts the orange in half and gives each child half. The children are now screaming even louder than before. The sister wanted to present the wedges nicely on a plate and share them with her friends. Her little brother, on the other hand, didn’t actually want to eat the orange, but rather peel the skin off in one piece in order to do something creative with it.

Father’s solution only made the quarrel worse. Fair Share would have been adequate and simple: sister, the pulp, and brother, the peel.