Food
As a prince, Sanji was served the best food available in the kitchen. And the little boy was fascinated. So, he sneaked into the kitchen to watch the cooks work, watch how under their skilled hands' meat, vegetables, and other goods turned into the delicious pieces of art he saw every day on the dining table.
And Sanji wished to create such delicious food himself. That's why he took one of the long kitchen knives and a bit of meat one day and tried to cook.
Sanji wants nothing else but to cook delicious food, preferably for his family.