Subject: Why does inertia exist, what is the atomic explanation?

Newton stated as one of his laws that all matter has the property of inertia, 
the resistance to change motion. This is why objects accelerate when force is 
applied, instead of just right away recieving the motion of the source. 
I know it is common sense that objects accelerate when they get pushed. We 
observe this on a daily basis. But, if we could not observe, how would we 
arrive at this conclusion anyway by theory? Newton just states a law, not a 
scientific reasoning as to why it should exist! I would appreciate your 
response. Thank you for reading this.  

Re: Why does inertia exist, what is the atomic explanation?