According to "The Chimney Sweeper" by William Blake, the cause of Tom's suffering are harsh working conditions.
When he was just a young boy, his parents practically sold him so that he could start working as a chimney sweeper as soon as he got a little bit older. It is a very dangerous job, and he has to work from early morning, when it tends to get really cold until the very night when they let him rest a bit. Many children die because of these harsh conditions, and Tom is having quite a hard time himself.