Cushions should be comfortable. Soft, squishy, perhaps with a permanent impression from constant use. You should be able to sink right into its cozy familiarity. That is what comfort is all about.
Too much comfort, however, can come at a price. Sitting in that favorite armchair for long stretches can make your body as soft as that cushion. Not to mention the strain on a back that is in a constant slouch.
Minds are not cushions. As nice as it is to occasionally enjoy the comfort of the familiar, challenge is what keeps a brain strong and active. When my kids were younger, I always had to poke and prod them into more difficult areas. Why learn multiplication when they know how to add? Why read a novel when there are picture books? Time and again I would need to give pep talks to encourage them to move into unfamiliar territory, out of the comfort zone. I would remind them that tasks they found easy were once difficult. The challenge eases with practice, just as those new denim jeans soften with constant wear. Keeping mentally active is as important as keeping physically active. I read this recent study about how important it is, even for adults. Learning and doing new things rewires the brain, creating new pathways to make a more efficient thinking machine.
Comfort has its place. After struggling with a puzzling math challenge or science experiment, it's nice to sink back into familiarity with a simple story or a favorite DVD. The point is to avoid getting too comfortable.