The Inner Child & The Drill Instructor
On numerous occasions my oldest daughter and son will clean up a room or help out with a chore without ever being asked. However, the second they are asked to do one of these chores, they frequently respond, "It's too hard! You ALWAYS make us clean up everything! I can't do it all by myself!!"
It struck me that this phenomenon is mirrored in the avodas Hashem of adults as well. While it may be easy for us to choose to adopt new minhagim or take on additional chumros, we find it increasingly more difficult to do an every-day mitzvah simply because Hashem commanded us to do so.
The inner-child inside us only wants to do what it wants to do, not be forced to comply with rules; it rebel's against "The Drill Instructor's" direct orders.
Just as a father is pleased when his children help out around the house without being asked, Hashem is also pleased at the times when we go beyond the letter of the law in our avodas Hashem.
However, if our avodas Hashem is only based upon our own choices and performing our will at the times we so desire, then our Father's nachas ceases to be nachas.