From solid to liquid to gas?

After being taught about phase changes for many years, I STILL dont understand why there are distinct temperatures at which a substance all of a sudden turns from one state to another. Why not this gradual gradient where a solid steadily becomes a softer and softer until it becomes a liquid, and then the liquid steadily becoming more and more like a gas. But no. Nature makes it so there are abrupt changes in state but I do not understand why? Can someone give me an explanation?