Here's why:

* Gravity pulls you downwards: Gravity acts on your entire body, pulling you towards the center of the earth. When you're on an incline, gravity's force is working against your upward motion.

* Resistance to motion: This downward pull of gravity creates resistance to your movement upwards. You need to exert force with your legs to overcome this resistance and move yourself uphill.

Other factors that contribute to the resistance force:

* Friction: Friction between your shoes and the surface of the incline can also create resistance.

* Air resistance: While usually less significant, air resistance can also play a small role, especially at higher speeds.

In summary: The main resistance force you experience while walking uphill is gravity, which acts to pull you back down the slope.