myungsoo saw the girl, fell for her, imagined exactly how their relationship would play out in his head within 60sec which apparently created this alternative time line where that all actually happens. he sees that it ends in a break-up, so he gives up without a fight and leaves the ring for the girl. when the girl touches it and looks at him, she sees the imagined relationship too (?). and the ending is the parallel between "what if" they got together vs myungsoo staying alone. or something.