Jeff isn’t the nicest kid around, which is apparent even before Jessica Feeney arrives in their class. For example, he refers to those who struggle to read as “the dumb group.” Also, he regularly lies and acts out, over and above never really seeming to listen to Tom when they talk.
So, why would Tom want to be friends with a boy like Jeff? Tom Bender is the kind of child who could be described as a wallflower. He’s chubby and reserved, which makes it difficult to make friends. His extreme shyness is in stark contrast with Jeff’s outgoing, forceful nature, and it seems easier for Tom to hang out in Jeff’s shadow than to step out and make other friends. However, it is apparent that when Jeff and his other friends start being unkind to Jessica, Tom is not impressed.
Tom’s character grows significantly during the course of this story, and after getting to know Jessica, he chooses to spend time with her rather than jumping into the Cobra with Jeff. This shows that now that Tom has had an opportunity to make friends with someone else, Jeff is no longer the center of his world.
In a nutshell, Tom and Jeff are friends because it’s easy for Tom. Since they “never really said much to each other,” there has never been any pressure on Tom to come out of his shell.