Objects don't change their velocity for no reason. Their direction of motion is constantly changing as they travel along their elliptical orbits. Moreover, unless their orbits are perfectly circular, their speed is also constantly changing, with the speed being greatest when the planet is closest to the Sun. For every action, there is an equal and opposite reaction. What Newton's Third Law tells us is that if object A exerts a force on object B, then object B exerts a force on object A that is the same size but in the exact opposite direction.
All forces come in pairs.
The warthog pushs down on my hand with a force of 1 pound. In order to keep the warthog stationary, my hand has to push up on the warthog with a force of 1 pound. The downward force exerted by the warthog on my hand is equal and opposite to the upward force exerted by my hand on the warthog. Suppose I remove my hand, so the warthog goes into freefall. Now, the Earth pulls down on the warthog with a force of 1 pound.