Why does Grisha Yeager have to love someone? In attack on Titan ep 58, Eren  Kruger says that Grisha has to love someone inside the walls or else “history will repeat itself”. 

Why?
Probably "love" means have a child.  Mold the child's thinking and the child inherit the attacking titan with the same ideals of them. Maybe if he doesn't pass the power, the power would be lost and some new baby would inherited the power. It could also mean that in the past, maybe the attacking titan was lost during a fight and then a new born inherited the power. Which could explain how Marley only had 7/9 powers,  Padaris had 1/9, the attacking titan was lost, and somehow Kruger was given that power. Pretty much no one knew that Kruger had Yrmi's blood in him.  Maybe that caused a problem and made history repeat itself. 
There are different translations, I recall only to get married start a new family, and continue the mission to find the royal family and their power. I thinks that the family was just to fit in better, and child to pass the power.