Spontaneity and the Decisive Moment

The photo above is one from a while back from Photoville. This photo captures the perfect moment of the man aligned with the frame of the structure. Capturing the "perfect moment" is the broadest and vaguest subject to express. How does one photograph the perfect moment? What makes a perfect moment? I have been having difficulties trying to capture a timing like this. Attempting this assignment I have learned that a spontaneous and decisive moment is not only technical but also a record of documentation of people's personal lives. When looking at a photograph from 50 years ago one may not know the people they are seeing but the viewer can tell the significance of that time. The photograph does not need to be explained to understand the emotion behind it. Encapsulating this moment should be able to covey the emotions the subjects are feeling and project those emotions on to whoever is viewing the photo.