What motivates a person?

This evening, my mom had made a wonderful innovative dish for dinner. Over the years she has learnt to make a huge number of varied dishes. When we were young she would go to cooking classes after sending four kids to school in the morning. She would come back by 11 and prepare lunch for the kids. She would then make snacks everyday for us so that when we came home we would have something warm to eat before we went out to play. Even now, she experiments with various ingredients and watches a number of cooking shows.
The fascinating thing is that if she was cooking for herself alone, she would make very simple quick dishes. I have never seen her cook interesting dishes just for herself.
What motivates a person to spend so much time and effort and learn and cook so many things just to give pleasure to others?