I think that the stranger is probably literally a stranger passing by; however, Whitman feels a connection to him or her, or at least wants to feel a connection to them. The message of the poem is the potential friendship or connection that we could have with each human being we encounter. We often seek companionship, affirmation, love, and closeness in others, but pass by so manyopportunities to have that relationship. So Whitman states "how longingly I look upon you,/You must be he I was seeking" for exactly that type of companionship. If you have ever sat in a crowded place and looked at people that you don't know, and wondered about their lives, then you can relate to what Whitman felt. He wonders if that stranger is the person that could fill the longing that he has for that closeness to another person. He feels of the stranger that "You grew up with me, were a boy with me or a girl with me,/I ate with you and slept with you, your body has become not yours only nor left my body mine only, You give me the pleasure of your eyes, face, flesh, as we pass, you take of my beard, breast, hands, in return." This potential connection with a stranger is the point of his poem, that we could be that close, that we might be missing out on opportunities of friendship every day if we don't take them.