Symbolism, Imagery, Allegory
There are all sorts of "doors" in Bud, Not Buddy opening and closing for Bud (and sometimes being locked tight right in front of him). In this story, Bud talks about the lesson Momma gave him about doors: "…no matter how bad things look to you, no matter how dark the night, when one door closes, don't worry, because another door opens" (5.47). At the time, Bud was confused about what she meant, but as the story goes on, he begins to catch on to what doors represent: opportunity, or choices about which way to go or which thing to do.
Bud becomes pretty skilled at going for opportunities that come his way. If a door opens, he tries to run through it. For example, when his search for Miss Hill doesn't work out the way he'd planned, he realizes that a door has shut, but he thinks, "That library door closing after I walked out was the exact kind of door Momma had told me about. I knew that since it had closed the next one was about to open" (7.41). Bud isn't talking about the actual door; he's talking about how his opportunity to find Miss Hill at the library had run out. But instead of focusing on the closing door, he looks forward to the next door opening.
And that happens the next morning, when Bugs finds Bud and invites him on his journey west. It's an open door, and Bud accepts it. Then, of course, another door closes when Bud doesn't catch the train, but instead of wasting time feeling down in the dumps, he comes up with a new plan, an even better one: to walk to Grand Rapids to find his father (wide open door!).
Doors play an awfully big role in Bud's success and ultimate happiness. We bet you never thought doors could be so cool, huh?