I'll let you watch the video first, then explain what made it so powerful and startling.
The man in the video is Ulay. He and Abramovic first met in 1976, after which he, a West German performance artist, became her regular collaborator. He was more than just a collaborator, though: He was also her lover and muse.
Together, they designed performances that took away their sense of self, that melded the two of them into a powerful one, performances such as the quite literally breathtaking Breathing In/Breathing Out, where they sealed their mouths against each other, breathing in each other's exhaled breaths. Eventually their lungs filled with carbon dioxide from each other's exhalations, at which point they passed out, unconscious.
Of course, as such performances continued to push boundaries and ideas of who and what the self is, they eventually reached a point at which they could no longer go any further together, either personally or professionally. Recognizing their partnership had run its course, they parted in 1988 by walking toward each other from opposite ends of the Great Wall of China, then meeting to say goodbye in the middle.
Which brings us back to the video above, from opening night of Abramovic's 2010 retrospective. When Abramovic looked up for the next participant, the man she saw was no stranger, but rather her former partner and lover, Ulay.
As you can see, Abramovic was at first jolted--but their reunion proves a deeply touching scene.
*This content was inspired by an amazing article that can be found here: http://educateinspirechange.org/inspirational/sat-many-strangers-day-man-shows-got-goosebumps/