As a river will seek the route of least resistance, left to our own devices we will do the same. It is much more pleasant to play music which is easy from either being not very challenging or has been previously learned. Trying something new or especially hard requires much more discipline than playing a piece learned many years ago.
This also applies to difficult sections within a piece. A difficult passage may require four, five, ten or a hundred times as much time and effort as the rest of the work. If that extra effort is not made, the passage will never become as easy as the rest of the music. Unless it is attacked early and hard it will always be the weak spot within the larger work. We all know where these problem areas are; if we start with them and work on the “hard stuff first” the piece will progress evenly and quickly without the constant drag of the “hard stuff”.