As a young orphan who's been moved around from one foster home to another during his short life, it's not surprising that Bud's personality is initially somewhat passive. Like all children in his situation, Bud is someone to whom things happen, not someone who makes things happen. He's pretty much all alone in the world, a powerless child whose welfare entirely depends on the kindness of strangers.
At a young age, Bud learns a number of strategies that help him get through life. These life lessons are based on the understanding that a young orphan in his position needs to adopt a posture of passivity and gratitude towards adults, even when they treat him abominably. This how Bud initially behaves while staying with the abusive Amos family, who treat him appallingly.
Yet over time, as Bud matures, he starts to stand up for himself. Sick and tired of the abuse he's encountered at the Amos residence, he leaves and vows never to return to the orphanage. It would appear that Bud has experienced something of a character change. Now he realizes that being passive is no longer a viable strategy; he needs to take his destiny into his own hands and be more independent and assertive. Put simply, he needs to stand up for himself.
As Bud grows more independent, he starts to become more active in the world. No longer buffeted this way and that by the winds of fate, he feels confident enough to head off to Michigan in search of Herman E. Caldwell, the man he believes to be his father. Thanks to his initiative and his change of attitude, Bud is able to take a giant step forward on the road to manhood—a road on which he's traveled some distance by the end of the story.