How can you be genuinely kind if you are not?

I think an example might be, if you see someone say, struggling with a door, and they have a big bag in their hands, at a store or wherever, and you are going by, to be kind, would be to see, 'hmm I know that is hard for me and 'I'd" like a hand if I were them' and then offer to open the door. I think it just comes down to seeing yourself in another. I love Byron Katie and her deep philosophical work.. she says 'When I look in a mirror, I see you'...meaning we ALL have thoughts, feelings, and wants, we all suffer or feel a bad day; so it's really just helping 'you' in a way when you are kind to another.... it is almost like you were being kind to you- when 'you' were in that situation. That's just an opinion, I'm no expert on kindness.... and people are RARELY kind to me, as in go out of their way to smile or hold door etc. But, it occurs sometimes, and I think, 'wow that was really nice' (for ex. someone came to a stop on a busy road to let me out of a side street ... there was no traffic for them to already stop in, so they did it just to be kind. I appreciated it)