Normally things tend to fail if we don't attend to them i.e. a
pile of sand remains a pile(chaotic/meaningless) until a child turns it into
castle, the tendency of material is thus to remain in its state of chaos but
evolution seems to be making organisms better and better.What is this force
driving the process of evolution?
A sand pile is not chaotic and meaningless. Chaotic and meaningless would be for the sand grains (in fact, their constituent subatomic particles) to be distributed randomly and more or less evenly throughout the universe.
There is no limit to the things in nature that are not chaotic and random. Stars form, planets aggregate around them with their heavier elements toward the center, oceans collect on the surface, deposits of minerals appear through various processes, plants and animals grow. The force that drives most of these processes is our Sun
, which is constantly pumping unimaginable amounts of energy into our system. Evolution is one of those processes, so ultimately, as with everything else, it is driven by the Sun.
But if you mean what causes evolution to take one path and not another, think about what makes a stream take one path and not another. The stream always flows downhill. Even though its immediate path might be blocked by a rock or a branch, it will flow around it, always ultimately downhill. Gravity is the pressure that pulls the stream downhill.
Evolution has many pressures: changes in climate or geography, changes in the numbers or strength of predators or competitors, etc. But always evolution flows in the direction of the survival of the genes, because if it didn't, we would not be here today to talk about it, would we?
Just like a stream, there are many branches of evolution that dried up and disappeared. We happen to be one branch that hasn't - yet. If that's the definition of "better" then you could say that every species alive today is "better" than the ones that died out, and all species are equally "good".
edit: Here are some pictures of some amazing formations
that look as though they could have been formed by some intelligence - yet they are simply natural.