Why? You only reach this conclusion because everything we experience has a beginning and end. In nature, we correlate events in 'cause' and 'effect' in order to logically interpret them. This is instinctual, but as we study the universe more, the need for a cause to an effect proves to not always be the case. The universe was once infinitely, uniformly dense energy. Why did it clump together and form particles and forces? Because it could.