"I am he as you are he as you are me and we are all together" — The Beatles
Once one has learned to kindle the feeling of metta towards oneself, the next step is to extend it to others. The extension of metta hinges on a shift in the sense of identity, on expanding the sense of identity beyond its ordinary confines and learning to identify with others. The shift is purely psychological in method, entirely free from theological and metaphysical postulates, such as that of a universal self immanent in all beings. Instead, it proceeds from a simple, straightforward course of reflection which enables us to share the subjectivity of others and experience the world (at least imaginatively) from the standpoint of their own inwardness. The procedure starts with oneself. If we look into our own mind, we find that the basic urge of our being is the wish to be happy and free from suffering. Now, as soon as we see this in ourselves, we can immediately understand that all living beings share the same basic wish. All want to be well, happy, and secure. To develop metta towards others, what is to be done is to imaginatively share their own innate wish for happiness. We use our own desire for happiness as the key, experience this desire as the basic urge of others, then come back to our own position and extend to them the wish that they may achieve their ultimate objective, that they may be well and happy.
http://www.accesstoinsight.org/lib/auth ... toend.html
It cannot be stressed enough the importance of identifying oneself with others. Some people tend to think that they are the only existing person in the world and that another being is just "someone out there" — someone who doesn't have feelings and therefore we can hurt them if we so desire. But if a person reflects deep enough he will see that every living being is just like him.
Every being is just like you. And me. They all have their own body and mind. They all have five aggregates and six senses. They all have their joy and sorrow, their successes and failures, their mistakes and their good virtues, they are all subject to the suffering of samsara and they all experience the pleasure of the human realm, which after all is one of the fortunate realms. They all have the same physical organs and the same mental faculties. They all sleep like you do, eat like you do, go to the toilet like you do. We all have roughly the same daily programme and face the same challenges. We all want to be liked by others and most of us care for one another. I can continue listing the things we share and are common to all of us, but I think you get the point I'm trying to make.
The point I'm trying to make is that you are the centre of the Universe. And so is every other living being. Every being is the centre of their own Universe.
You might be walking down a street. You clearly perceive how you stand at the centre of the Universe and you see many other beings walking past you. They count as well. We are all Universes walking past each other.
I recently had a stuffy nose, for about a week. Shortly after, my mum caught it too. I said "Mum, maybe I've transmitted it to you." And she said something to the effect of "I'd rather have a stuffy nose than see you have a stuffy nose". I thought that's not fully compassionate, you count too, mum! But she continued "Do you know how many times I've wished that I suffered if that could take away your suffering." Sounds Mahayana to me. And that may sound like compassion, and it kind of is, but.
But you count too. As Ajahn Brahm often says "It's not about you, it's not about me — it's about us!"
A little exercise: imagine you're posting a post on a forum like this one. There's a poster above you and there's a poster below you. Now imagine you are the poster below you and this other guy or girl above him/her (you) is just someone else. Now that's identifying oneself with others in practice. And you can do that in real life too! Imagine you are your boss and your boss is you. Would you ever despise your boss for giving you a hard time?
By identifying yourself with others you will care more about them. You will never think of hurting them. How can you? Would you hurt yourself?
Look into a tall mirror. Is that another being there or is that you? See all other beings as a mirror of you.
I'm not perfect myself but I'm trying to try my best.