I like circuit workouts. They are efficient, flexible, and challenging. But if that is all I ever do, my body will get bored. Bored bodies, like bored children, tune out and stop learning; they may even start whining about how they want to watch television or eat more cookies.
Sometimes I need to choose yoga to calm my mind, stretch my muscles, and challenge my balance. Sometimes I need to lift the heaviest weights I can to increase my maximum strength. Sometimes I need to go as fast as possible and sometimes as long as possible. Sometimes I need to try something totally new to shock the system, allow myself to be bad at something, and laugh.
Routine is a very powerful motivator and I would not want to suggest that anyone give up their Tuesday golf game if that is what gets them moving. If the habit is to go to the gym on Wednesday nights, maybe a date with the treadmill instead of the bike could spice things up, or the weights instead of the usual zumba.
When I switch it up, I get to grow. And sometimes I get a great story about that time I fell off that new piece of equipment and got a Superman bandage, too.