You are both trying to paint different things on this canvas. Maybe the top person wants to post his hand on your chest to stand up or posture. You need to get his hands off your canvas. But maybe you want to armbar him, so you drag his arm across your canvas, or better yet you drag it so well you can take his back.
Bottom guy goes for your collar, your bury your head into his chest. Bottom guy wants to go for a triangle, you bury his head into your chest.
You are trying to pass, you post your hand on his hip. He is trying to pass your guard, you frame so he can't grip anything on your torso.
Because of gravity, it becomes the bottom person's torso that becomes a canvas. You guys are both trying to draw your masterpiece on it. Guard was invented so the bottom guy can actually use his torso as offence from the bottom, as long as he is the one in control of it. Top guy must draw his own masterpiece on it to defeat the bottom person.