Observe, but don't intrude.  A good photographer keeps his/her eyes open at all time constantly observing his surroundings and taking in the visual environment.  Observe, but don't intrude, is a good rule of thumb, as in the case of these two lovers along  the Seine river in Paris.  By shooting from a comfortable distance with a short telephoto lens, the photograph has a timeless quality -- and captures a moment instantly recognizable to all romantics.  If the photographer had tried to get too close, and inserted himself in the situation, the subjects would likely become self conscious, and the moment would be lost.