Can someone tell me the plot why theu dying? I dont understand anything
She died due to accidentally falling into train tracks. Dude killed himself out of grief.
Goddess of Love gives him a chance to go back to a time she was alive to fullfil his love; but she is destined to die without intervention. This intervention comes in 2 forms: the gun which sends him back using his lifetime as ammo, and lovey-dovey stuff which increases her lifetime.
At this point, they have to sleep together to continue living the rest of the life remaining to them. But they haven't, so are burning up their remaining time fast.