When I deliver the bill to my customers, I only give the good candies to customers I like.
That doesn't seem fair. Why do you only give good candies to customers you like?
It's my restaurant and I can do whatever I want to.
It's normal to want to reward people that you like, but it's not fair to punish people that you dislike. Sometimes people can just be having an off day, you shouldn't judge them too harshly based on a single encounter. Try to be kind to all of your customers and be patient with the ones that aren't so nice.
That's too hard for me. I don't think I'm a patient person.
I can understand it being hard to be patient with unlikable people but it's a good skill to have especially as an owner. You can try practicing by first being fair with everyone.