One of the most formidable difficulties in the service of God is routine–the Achilles' heel of anyone who fears God.
At a certain point, everything becomes unremarkable.
And at that moment, one is no longer serving God.
A person may be able to do things that, in the eyes of others, are unimaginably difficult; for him, nevertheless, they are not avodah ("service of God"; literally, "work") at all.
When we see someone with a full beard and dressed in black poring over the holy books day and night, there is no reason to assume that he is serving God more than his neighbor who wears a dime-sized yarmulke and is seemingly distant from observing the commandments on a high level.
One who reaches a certain level of effort, becomes accustomed to it, and allows this to be his norm, he is no longer one who is serving God.
Perhaps he was once.
–Rabbi Adin Steinsaltz
From Learning from the Tanya, Chapter 15, by Rabbi Adin Steinsaltz