He learns that material items and riches are not the most important things in life.
He learns to appreciate the little things, such as animals and gifts from others (material or otherwise).
He discovers a newfound appreciation for his friends.
He learns that sometimes one must be brave and strong to overcome their obstacles.
He learns that trying to swindle a dragon out of a mountain full of gold is often more difficult than simply making him expose his weak spot.
However, the greatest lesson Bilbo learns is to say yes to adventure, even if one is