Have you heard the story of the naughty little boy whose mother made him sit in a chair for time out?
He rebelliously told her, "I may be sittin' down on the outside, but I'm standin' up on the inside!"
We Christians sometimes do this too, even though Jesus makes it clear that our heart motivations and attitudes matter (Matthew 5:21-48).
Jesus cares what we're "doing on the inside."
Legalistic Christians do the right thing out of self-righteous pride or fear of punishment.
Weak Christians do the right thing until they realize it isn't popular, easy, or fun.
The legalist misses the reason for obedience: our love for Christ.
The back-slider misses the reason for God's commands: God's love for us.
"Perfect love casts out fear;* it also casts out sin, and becomes the motive power of the whole spiritual man." — Dougan Clark, The Theology of Holiness
* 1 John 4:8