The real question would be, why do charges exist, and why only positive and negative? Why don't we have charge A, charge B, and charge C? This is something rather hard to answer and can be questioned indefinitely. You'll probably need to ask a string theorist :P
I believe we came to the discovery that if we take "charged" objects and put them close together they would repel. Sometimes they would attract. Then at one point, a stroke of genius hit man, and voila we have the positive and negative charge that repel like charges and attract opposites. It has nothing to do with lowering of energy. It's just a fundamental concept of our universe. Another question to you: why do masses attract each other? Why do masses exert gravity?