De Sawpedia :
Zep formed a sort of bond with John [Kramer] while he was being treated, but was considered to have "issues of his own" by him. Zep talked about the doctors behind their backs, saying that they were all having affairs and calling Dr. Lawrence Gordon a cold-hearted bastard.
John seemed to see a lot of his own actions in Zep's goals, reminding him of himself before he was diagnosed with cancer.
The reason why John decided to test Zep was because the former noticed that Zep was wasting his life with dreams of becoming a doctor, dreams that were never going to come true.
[Saw: Rebirth]