When he was young his father gave him a seedling tree to look after and grow. His attempts failed and the tree died, he tore it out of the ground out of frustration. Lucas Rockwell was a married man, his wife's name is never disclosed and he mentions to Ben during a scuffle that he had kids. Rockwell lost his family during an Indian raid, they murdered and scalped them and since then he's had a strong hatred to Indians. It is here we find out Rockwell only agreed to come along so he could watch Ben fail at his dream and lose his wife and children to it.