Ok, here's my take. I don't think Reiji believes he deserves happiness. He is all about karma. He believes that the only way for him to be "set free" is by taking responsibility for the pain and death he has caused (Cal included). The only thing that he wants to do before he dies, is to keep his promise to make Eren smile. Eren is looking for a way to be "set free" as well. She doesn't know where she came from or have any memories. The only thing she has is Reiji.