Why did Tom want to fight with the new boy

First, the boy is the "new" kid. Second, he's bigger than Tom, thus, Tom has something to prove. Third, his clothes looked a little bit better everytime Tom gave them a good look...... Tom fought with the boy because "boys will be boys." No specific reson, he was simply looking to pove himself.

"He had met a stranger, a boy a little larger than he was....The boy’s clothes were new and good, and he was wearing shoes. Tom would wear shoes and good clothes like these only to church on Sunday. Tom looked and looked. The boy’s clothes seemed to become better and better, and his own clothes seemed to grow poorer."