Monkey went to the hospital on a lark, and entered the doctor’s room. At that moment, the doctor wasn’t in, and he’d left his [doctor's] clothes in the room.
Monkey thought this would be fun, so he put on the doctor’s clothes, that white lab coat we see so often. Just then a little bear came to the hospital to see the doctor. This was the little bear’s first time at the hospital, and he didn’t know what hospitals were like [lit: didn't know the hospital situation]. All he knew was that doctors wear white lab coats.
The little bear had shown up at the hospital carrying a basket of peaches (this was the payment he had prepared to give to the doctor), and he happened to walk into the room where Monkey was. Little Bear didn’t know Monkey, but he recognized the white lab coat, so he asked Monkey to examine him.
Monkey originally just [intended to] have a bit of fun, but when he saw Little Bear’s basket, he coveted it fiercely. So Monkey put on a big show of examining Little Bear, and accepted his well-earned basket of peaches. As to whether or not Monkey actually cured Little Bear, we will never know.
However, after Monkey tasted that sweetness, he often stole into the hospital, taking advantage of the doctor’s absence, and examined patients. Later, Monkey simply made himself one of those kind of white lab coats.