Full Idea
To require that duty always be somewhat burdensome - as Kant does - amounts to acquiring that it never become habit and custom: in this requirement there linger a tiny remnant of ascetic cruelty.
Gist of Idea
Seeing duty as a burden makes it a bit cruel, and it can thus never become a habit
Source
Friedrich Nietzsche (Dawn (Daybreak) [1881], 339)
Book Reference
Nietzsche,Friedrich: 'Dawn (Daybreak) (v 5)', ed/tr. Smith, Brittain [Stanford 2011], p.200
A Reaction
Habit, of course, is the ideal of Aristotelian virtue.