Defending the title

As poor as City have been in defending the title it's interesting to see how the champions of the other top leagues are faring.
Juve are on course to retain Serie A but Dortmund are adrift by 20 points and Madrid 13 points.
Looking at the PL history out of the 20 years its only been retained a handful of times,Chelsea once and Utd 5 times
Wonder what makes it so difficult the following year,is it a mental thing where the players switch off and can't regain the same winning spirit of the previous season?

Admin can you multiboard on the Utd,Chelsea and Arsenal boards?
And just for a laugh include the dippers and Tottingham