So for Vito to bring him to heel so to speak, Vito had to be even scarier than we know.
I believe, it's something else. Imagine how it feels being Luca. Everybody hates or fears you. And then, when you're really down, somebody treats you like a friend, shows understanding and respect.
Vito made Luca important, he paid him more money than he could earn anywhere else.
Seems that Vito radiated omnipotence. (Even Genco believes in the end, Vito would be able to stop death.) So whatever Luca expected from life, he believed (probably rightfully) he'd get that from Vito.
