The problem in a life of steady devotion and sincere worship lies in the fact that habit makes the tension slack.
Piety can become a comfortable state of being in which one is afraid of change.
When arriving at such a stage, the sincere worshipper of God has to assume an additional burden, to tighten the tension.
Otherwise he will slip into a state of gradual spiritual decline.
–Rabbi Adin Steinsaltz
From Pebbles of Wisdom from Rabbi Adin Steinsaltz