Random
Why? People ask this simple question every day, but they can never get the real answer. You can follow the chain of causality until you reach a dead end, where pure randomness takes effect. At this point, there is no reason. This is also where the Universe is open to interpretation.
So what are the possible interpretations of randomness? I can see two main sides to this question. One could argue that it is the work of a divine force (God), and is the vehicle with which it interacts with the physical world. Others would counter that there is no evidence of this, and that randomness is just what it is (arbitrary). I argue that either of these arguments is valid, since the truth is unknown and unknowable.
This is something people need to understand when it comes to arguments between science and religion. For example, it is because of the above statements that evolution and creationism are not mutually exclusive. It is pretty obvious that every animal didn’t just appear out of nowhere as they are today, and that evolution does in fact occur.
The question that remains is whether the initial series of random events that created the first organism was under the control of some form of intelligence. This is something science is unable to answer.
So what is your interpretation?