The Beatles have this particularity to have a certain number of 'pivotal' albums. This term applies as well to "Sgt Pepper" as to "Let it Be" or "Revolver"... This is easily explained. Absolute reference in their glory, the four boys saw (and see) each of their release raised to heights of notoriety in which, inevitably, each change was meticulously taken note of by everyone: who writes what, it speaks about what, it is a concept, etc.
"Rubber Soul" is then the first of the pivotal albums. The experimentations indeed start to be felt: various instruments (the famous sitar on 'Norvegian Wood'), more unbridled structures which moved away from the very calibrated beginnings, and then 'Nowhere Man', the first song which does not speak about love. But it is especially on the small details of the life of the group that the rupture is felt. The Beatles started to consume all kinds of substances (via the influence of Bob Dylan), refused to be guided and took over their work in the studio.
Yet this wind of change is not immediately felt. In the form, the whole remains rather smooth and pop. As a result, all this positive energy is used to release some of the most beautiful songs that the band will do: 'Girl', 'Michelle', 'Norvegian Wood' in the ballads section, but especially some very energetic pop/rock songs: 'Drive My Car', 'If I Needed Someone', and especially the bouncy 'Run For Your Life'. All is millimetre between the vocal harmonies, the melodies, the energy, the clarity of the instruments... No false note, it's almost scary.
It is besides where "Rubber Soul" is an album which will really upset the career of the Beatles: it is too perfect. At the top of their know-how, the band members reach a kind of point of no return. No more doing the same thing after that. No more innocent pop. They will never be able to do better in this style, so they slam the door of their past and open a huge breach towards "Revolver". With the release of "Rubber Soul", the chessboard was in place and the Beatles' destiny lay before them. We know what happened next.