So in the original contract - it was that DOOM had to be brought on the world or she loses the person she LOVES the most. She told him before she knew him (when they were standing on her balcony) she was going to make sure that was him - the one she loves the most - because she didn't want to lose her aunt or brother etc.
She apologizes to DOOM after she actually FALLS in love with him that he is the one she loves. He is taken away from her (his choice) and made that contract VOID. He wanted to save her. He wanted her to live even though he knew he had to leave. (he is DOOM a deity so he didn't exactly die) Does that help? It is a lot about growth and sacrifice and love in my opinion - of course this is my opinion everyone takes away different things.
