I don't think we ever can. To do that we must be that other person. And we can't be that because we are uniquely ourselves. It's an inescapable fact of human life that we are isolated one from another within our own physical bodies and within our own private minds. However this existential state need not be a depressing thing. For we do have relationships.
I thought I'd turn to see what one of my favourite philosophical and spiritual thinkers, Krishnamurti, had to say on the subject of Relationship and Isolation. In The Problems Of Living he writes that Life is relationship, which is expressed through contact with things, with people and with ideas. In understanding relationship we shall have capacity to meet life fully, adequately... Relationship, surely, is the mirror in which you discover yourself. Without relationship you are not; to be is to be related; to be related is existence.
Examining our relationships, Krishnamurti recognises that many of our relationships are isolating because they are based on a desire for power and self-gratification, whether individually, racially or nationally:
Now if we examine our life, our relationship with another, we shall see that it is a process of isolation. We are really not concerned with another; though we talk a great deal about it, actually we are not concerned. We are related to someone only so long as that relationship gives us a refuge, so long as it satisfies us. But the moment there is a disturbance in the relationship which produces discomfort in ourselves, we discard that relationship. In other words, there is a relationship only so long as we are gratified. This may sound harsh, but if you really examine your life very closely you will see it is a fact; and to avoid a fact is to live in ignorance, which can never produce right relationship. If we look into our lives and observe relationship, we see that it is a process of building resistance against each other, a wall over which we look and observe the other; but we always retain the wall and remain behind it, whether it be a psychological wall, a material wall, an economic wall, or a national wall.
After forcing us to consider these uncomfortable truths, Krishnamurti then asks: Can one live in the world without the desire for power, for position, for authority? and replies to his own question like this: Obviously one can. One does it when one does not identify oneself with something greater (my italics). This identification with something greater - the party, the country, the race, the religion, God - is the search for power. Because you in yourself are empty, dull, weak, you like to identify yourself with something greater. That desire to identify yourself with something greater is the desire for power.
He concludes: Relationship is a process of self-revelation, and, without knowing oneself, the ways of one's own mind and heart, merely to establish an outward order, a system, a cunning formula has very little meaning. What is important is to understand oneself in relationship with another. Then relationship becomes not a process of isolation but a movement in which you discover your own motives, your own thoughts, your own pursuits; and that very discovery is the beginning of liberation, the beginning of transformation.
You may think we're deviating a long way from talk about how one individual can communicate with and get to know another individual, to ideas about power struggles and religious, national and political identity. But relationship is relationship, and we're talking essentially about the same relationship here - the individual one-to-one personal relationship being a microcosm of all the other more global, more abstract relationships.
The technique Krishnamurti uses in the above "argument" is typical of his reasoning; and, I think, typical of certain kind of Buddhist mode of thought.
I've written about relationship before in my post We Are Stardust, We Are Golden.