That's kind of the point. Ask yourself: which people would you genuinely be excited to make a little happier? (through a compliment or otherwise) Whose opinion are you keen to carefully listen to and consider? Who do you like enough such that you will want to put in the effort to remember their name?

I think the idea is that if the stranger on the bus has a haircut you genuinely find to be wonderful: tell them about it. You don't need to force yourself to be nice, just take action on the things you're genuinely excited to do.

And if you don't ever want to be nice to people, then you have some digging and reflection to do (including about if/when you are nice to yourself).