He gave her a couple of drops of the love potion when they first met as an experiment.

He doesn't know that she took all of the love potion and fed it to him in the bentos she would make for him. (Hence why her bentos are "filled with all her love"). As to whether or not the potion influenced her to do that is debatable. (I lean on the side that she's just psycho to begin with.)

So when you see him rushing to see her at the ending, it's because the love potion just took into effect. (It takes 3 days for it to happen.)