Gravity, is one main factor while riding a bike. When you are to go down a hill, gravity is doing all the work for you pulling the front of your bike down the hill. Gravity will always be greater than the friction, and weight you are fighting through. Friction when going down the hill is very minimal between the spinning tire and the ground. But when you are going up the hill you are doing a lot more work because you are fighting between friction, gravity, and weight.
Gravity will always be something you have to fight through no matter what your doing because it will always be there.