Weather is different in different places at different points in time. It constitutes wind, temperature, humidity, atmospheric pressure, sunshine and precipitation. On the basis of these factors, it differs from regions to regions and even on daily basis.
The earth rotates, thus causing wind to move various weather patterns in and out of different areas. Factor in the types of changes that occur, as mentioned by aicha, and all these affect the changing of the weather, often day by day
The weather changes because it does.