Frisk was forced to pretend to be Sans' girlfriend because it was the payment of her to make him leave her house and of course, to protect her eardrums. Sans really likes songs which the singer just shouts and shouts. If she really didn't swim in the pool of stress, she would let Sans live forever in her house. Frisk also needs the money from Sans' rent to repay his dad's debt. In their 'pretend to be a couple', Frisk slowly knew more about him. There was a sweet side to his personality. He also loves his mother dearly. What would be the feeling of being loved by Sans if she really was his girlfriend? Frisk got butterflies in her stomach on that idea. But she almost fainted when Sans said that they would pretend that they are a couple seriously. But one truth would tell the tale that Sans couldn't love her no matter what. So what are you waiting for?