A pendulum is a component on a clock that swings back and forth with every second in time.  It is often used to refer things in life swinging from one extreme to the other.  If you have a scale where on one side is John's distaste for his monotonous life, and on the other side is his regret, longing, and craving for that life back, then John swings from one side to the other quite quickly.  So, John himself is like the pendulum, swinging back and forth between two opposite extremes, very quickly.  You would not expect, that just seconds after his wife returns back home, that he would switch from feeling resolutely determined to be a better man, from fiercely longing for his wife, to wanting to immediately leave, returning back to his old ways.  So, yes, John and the pendulum are very much alike, and that is the reason for the title itself.