What is it about the human mind that makes routine comfortable?

The mind is the most "powered" organ of the body. It dedicates all that energy to conducting a lot of functions. 90% of brain activity is automatic and subconscious functions. So often, the brain cheats and cuts down on its energy requirements by forcing the conscious mind to economize. That often boils down to creating a pattern of behavior and cognition that requires little deviation of thought by following a predetermined plan. If you drive in a straight line at a constant speed, you use less fuel than you would making turns or accelerating.

It's nothing personal, but your subconscious mind doesn't like it when you think too much. It actively works against you when you do. Thinking hard is a struggle and a fight with yourself. Routine is the subconscious mind telling the conscious mind to stop putting up a fight and just do what it is told. Farmer versus warrior. Logically, in a routine, it's dominance of a pattern. Not control.