When novice riders start a new method, then - after practicing the basics a few times - they often say that 'their horse gets bored'.
Now most of the time it's a projection of their own 'boredom';
When a rider is only practicing the 'names' of the exercise and not really the 'essence', that might lead to loss of interest.
And when a rider is only concentrating on the WHAT of an exercise and not on the WHY, that might lead to a 'dull' training.
Boredom also comes from if you start to compare with yesterday and when you start to expect, when you stop having an open mind for all the tiny details, when you stop being just interested in what happens, or when you stop having a beginner's mind.