If an object is at rest (\vec{u}=0)at one reference frame, e.g. with respect to you.
I think you would accept that the NET force acting on the object is zero.

Assume there is another observer which is moving with constant velocity \vec{v} with respect to you.
You saw another observer is moving with velocity \vec{v}

Let's back to the previous object, the other observer should find there is no NET force acting on that object.
And that object is moving with \vec{u'}=-\vec{v} with respect to that observer.

From physics point of view: The physics law is the same for all inertia frames, so they all find the same force (or no net force).

An object moving with velocity \vec{v} will be seen as at rest for another observer which is moving at the same velocity \vec{v}.

Assume you site in a train which is moving at constant velocity \vec{v} with respect to ground:
For all observer on the ground, you and other people in the train moving with constant velocity vec{v}
However, for observer in the same train, all the others are not moving at all ,right? ;)