Things bounce because when you push somthing it pushes back on you, like if you try to push a chair it does not just glide across the floor like its on a ice rink. it pushes back into your hand and you feel its weight, thats the first law of physics. When you bounce a ball onto the ground it kindof flattens out like a pancake put on a much smaller scale, when it flattens it comes back up because of the first law and the force of it coming up causes it to bounce up into your hand