[Peach Blossom Peak, Near The Bamboo Forest]
There are three possible ways of making the beasts willing to follow a person.
One is defeating the beast with your strength and letting the beast recognize your power, then the beast is willingly to submit and serve. Two, the person might have the life-saving grace to the beast that made it feel grateful and repay back the kindness by submitting itself.
The third one is…the beast took the initiative to follow willingly.
It's just that, the second method rarely exists. Even if that person had a really heavy life-saving grace, the beast won't become your contracted beast easily. Some beasts might choose to offer other priceless treasure or other method to repay back.
Becoming a human's contracted beast means that the beast has to lose its freedom. Who wants to be other people's slave and lose the freedom?