Tarouza, as a baby, had lost the reason to live after hearing from his real mother that he shouldn't exist. Lucky for him, Monoko was there to be his mother.
The lion cub's despair when they realized that their mothers would let the other lions kill them.
Many animal's backstory and the reasons of why they decided to follow Tarouza.
Catherine the hippo lost her child and no one in her herd helped her. To rub salt in the wound, when she expressed her anger at them, they drove her out.
Dougen the tiger lived a lonely life because he was too strong and the other tigers didn't want to share their food source with him. He managed to adopt a deer and saw the child grow up and had a family of his own, but then a tiger came and killed all the deer. In his rage, Dougen killed that tiger only to discover that it was his son.
Tarouza showed Dougen that his son had a child. This made Dougen swore to bring the human to the top of the tower so that his tragedy will not repeat again.
Monoko sacrificing herself in order to protect Moko and a wounded Tarouza from being eaten by a dog that wandered into the animal village. It's all she could do despite the fact they have the Eternity Fruit, she can't talk the dog into eating it instead of her children. The tragedy of her death is so shocking that it affects everyone. Little Moko for one, can't grasp that her mommy is dead and she sets up a little tea party for her family in the hopes that Monoko would come back like she would always did whenever Moko cleaned up their home. However, no one is more affected than Tarouza. Monoko's death is the catalyst that finally drives Tarouza to completely change the Crapsack World they live in, regardless of the consequences.