You can play tennis for hours and hours. It is easy for you to do well and you enjoy it. But people get more satisfaction from doing the thing that they find most difficult. On the one hand, we like to do things that are easy. On the other hand, we also want the joy that comes from doing whatever is most difficult.
The grass is always greener on the other side. To accomplish something that is more difficult for us to do gives us much more joy. If we have a tennis ball to play with, we will start looking for a basketball, just because we do not have one. But if we had a basketball, we would start looking for a tennis ball. Again, some people will say that they need both a tennis ball and a basketball.
There is also another aspect to this. Even though we may want to run only 100 metres, we want to get the glory that comes from running a marathon. When somebody runs a marathon, he is appreciated and admired. So we feel that a marathon is more important than a 100-metre run. The most difficult thing impresses us most. This is human nature.