She wants to live and die as a human once and for all, not having a long and frustrating life with 000 (the demon king in the future). She's not used with the way demons there think lowly of life (like how they shrugged it off when the prince is killed on an instant, tho he deserved it but it's still shocking for Mary coz he's the only human she ever encountered again after so long).
They have different perceptions and living continually with them makes her fall into deep depression so she leaves the mansion to live on a far and separate house where she could age and die like how she's supposed to be (coz her being separated with 000 would mean the curse of long life/immortality would weaken). Though she loves him but I guess she loves herself more.
Ahh I see
Can someone explain what is happening!? ಠ_ಠ