Want someone to be loyal to you? You have to give them what they want, consistently.
No one is loyal to another person or organisation for the sake of it, although it sometimes appears that way. We are loyal to them as long as they are helping us fulfil what’s most important to us.
I’m loyal to myself. And you are you are loyal to yourself. We are all loyal to ourselves first and foremost, because we're all secretly (or not-so-secretly) self obsessed and do whatever we perceive will help us fulfil what we most value in life, or bring us the greatest advantage.
Don’t fall into the trap of expecting someone to be loyal just for the sake of it, or because you have some kind of agreement (like a marriage certificate). Loyalty only exists as far as someone is getting their needs met; hence we are all loyal to ourselves…
It’s nuanced and layered and doesn’t follow the conventional rules of what you think loyalty is.
If you want someone to be loyal to you, you must understand them, what they value in life and how they seek to fulfil what is most important to them. Then help them to get more of it, in a way that helps you fulfil what you value.
Create a mutually beneficial dynamic, the same way you would with a customer - keep them happy in a way that keeps you happy. Winner winner, chicken dinner.