Why is acceleration inversely proportional to mass?

The reason is because of ##F=ma##

If I rearrange the equation, I can see that ##a=F/m##

As the mass of an object increases, the amount of force necessary to keep it at the same also increases. Think about moving a piece of furniture, let's say a chair, from one side of the room to the other. Not too much !

Now imagine as you are moving the chair your freeloading friend sits in the chair! The mass you are trying to move increases, you are going to have to apply more force to keep the chair going at the same speed!

Inertia, or the tendency of an object to resist changes in motion cause acceleration and mass to be inversely related!

Easy, kind of entertaining can be on this site http://www.dummies.com/how-to/content/newtons-second-law-how-net-force-mass-and-accelera.html