When you shake a massive object, why does your body shake, too?
That’s because the massive object is shaking you. Forces always come in equal but oppositely directed pairs, an observation known as Newton’s third law of motion. So, if I push on this lead brick, and I shove it hard to your left, it pushes back on me equally hard toward your right. Two forces, in opposite directions; my force on the brick, the brick’s force on me. Now, this brick is pretty massive. It’s not as massive as I am, but it’s getting there. So I have to push very hard on it to make it accelerate away from me. It responds by pushing very hard on me, making me accelerate away from it. We’re shaking each other.
So, when you shake an object with very little mass, like this baseball, it’s pushing on you as well and shaking you as well, but it’s hardly noticeable. When you take something of comparable mass, like the brick, the shake is significant. I have to push very hard on it, so it pushes very hard on me.