Just remember, the force never just goes away, it doesn't disappear into nothingness. It has to go somewhere. So if I chuck a bunch of mass behind me, like if I am carrying fuel and I burn it and it leaves me by escaping as focused gasses, where does the force of the escaping molecules end up? Intuitively you probably know this, the top of the combustion chamber.

Have you every stood up in a canoe or similar boat and picked up a cooler and handed it to someone on the dock? What happens to the boat? It starts moving away from the dock a little bit. As I hand it to the person on the dock the mass that was in the boat has moved and as a result the force of my moving it is transmitted through my legs and into the bottom of the boat. The boat moves in the opposite direction of where I moved the object of mass.

The why to all of this is a deep discussion about gravity, mass, and the nature of time.