To a Stranger .

I think the poet is talking about an actual stranger, because in the poem he/she is explained as a random person passing by. The poet feels like he once knew this stranger; maybe from the past or another life because he says, "I have somewhat surely lived a life of joy with you." He wants to know this person again because of the line, "I am to meet you again." I think he wants to actually feel connected to this person the way he already is in his thoughts.