What life lesson does Bud learn in Bud, Not Buddy?

Bud, Not Buddy is a bildungsroman (or a coming-of-age story); therefore, it contains many life lessons for Bud. For example, Bud learns how to survive during the Great Depression. However, that is not Bud's most important lesson. In particular, Bud learns the true meaning of family.

The first way Bud learns about family is by experiencing ways that he does not fit within other particular families. First, the reader sees Bud being abused by the Amos family (Bud's foster family). Mrs. Amos does not even “hear” Bud over her own son (who can do no wrong). Further, being locked in a shed with a threat of a beating is certainly child abuse. The Amos family, then, is not Bud’s true family. Later, we meet the family that helps Bud at the mission. 

I watched them walking away. My pretend brother looked back at me and stuck out his tongue, then reached up and took my pretend mother's hand. I couldn't really blame him.

Even though the family helps Bud, they do not adopt Bud as their own. The children in the family are disgusted by having to share their sweets with Bud. These two families (as well as others) can be compared with Bud’s real family: the jazz band.

The most important way Bud learns about family is by finding his own family with the context of Calloway's band. The jazz band, even though it is not a “typical” family, is a perfect and “real” family for Bud. Bud is accepted and welcomed into this family. Further, he is given his livelihood by this family (his “skinny wooden flute” and his saxophone). Bud is also required to provide for the family in what ways he can. Specifically, Bud is asked to help clean and learn multiple instruments. In these ways, Bud finds the home he has been looking for within the jazz band.

What does Bud learn about friendship and himself in "Bud, Not Buddy"?

Bud gains perspective throughout the novel and learns many important lessons about friendship and self-identity. Bud learns that friends are instrumental throughout life, and their support can assist and encourage a person during difficult times. Bud's best friend, Bugs, journeys with him to the Hooverville and provides moral support, Lefty Lewis graciously offers Bud a ride to Grand Rapids, and Steady Eddie makes Bud comfortable around the band and Herman Calloway. Bud learns that friendship is essential in order to have a happy, fulfilled life.

Bud also learns that he is ultimately in control of his own fate, and that he has the ability to carry his mother's memory with him wherever he goes. He learns that things work out for the better for those who remain determined and keep a positive attitude. Bud also learns that he is actually Herman's grandson.

Throughout the novel, Bud lives by various "Rules," which are actually false generalizations about most people. He learns that each person is uniquely different and that he cannot make accurate generalizations about those around him without personally knowing them. Bud also learns that the majority of people are caring individuals who offer help selflessly.