:) Can any one tell me why earth rotates??

Can you tell me why it shouldn't? O_o

When things form, they have a rotation governed by how those pieces formed. If the pieces have (essentially) random distribution and velocities, then the formed body will also have a random rotation. The chance of a random rotation being 0 is essentially 0.

There are of course tidal effects and things, but these act over a very long period. For example the moon rotates exactly once with each revolution, because that's how the tidal effects make it stable to do so.

The Earth rotates just like everything in space rotates, because there's no reason for them not to.