How a Bodhisattva make the choice to keep suffering for others?

It's said Bodhisattva is very kind and he decided to keep in the Samsara until he fulfill his Pāramitā and become Buddha, so others can free from Samsara by listening to him. Even he could attain Nirvana at past Buddhas, that he didn't. **How he did it?** **What kind of thinking was there?** **How he decided keep suffering / keep live in Samsara, for others?** **It can't be just kindness. Can be?** Reference :- There's no linkage between people, they are just individual consciousness in vast universe. There's no actual family, friends. So how could there be kindness? Not normal kindness. But to play **most dangerous game** of, playing in the Samsara for saving others.