Why we feel easier to get downstairs and difficult and get tired while going upstairs
Going with gravity, compared to going against gravity....
because of elevation.
Because you still have good knees
Because you're resisting gravity. It's like going up and down a hill. It's much easier to roll down a hill than to roll up one.
Because we live on Earth, not the Moon.
Because the muscles used are not used significantly in plan level ground and the gravity resistance factor.
It takes more physical exertion to climb up stairs