Everything Has a Meaning, At Least According to Leibniz

I have recently began reading, "Plato and a Platypus Walk into a Bar" which teaches philosophy through jokes that state philosophical concepts quite clearly. While reading, the book introduced to me Gottfried Wilhelm Leibniz, a rationalist or philosopher who influenced the minds of many people with his ideas. Many of his ideas include religion, stating that everything happens for a reason. As the book states, his theory is the complete opposite of a joke where God is speaking to a man that has recently entered Heaven saying, "It's a bit embarrassing to admit, but everything that happens happens for no real reason."

If everything did happen for a reason, and if somehow the tables were turned and what was supposed to happen didn't, wouldn't we know? Wouldn't there be a series of repercussions resulting from that event? This brought me to the new idea of our own conscience. Throughout the day, we ultimately make our own decisions. We make the decision to wake up at whatever time we please, eat whatever we want when ever we want, and carry out actions of our choice throughout the day until the next. If we make our own decisions, then how is it that there is a more powerful reason behind me buying a small soda at McDonald's rather than a large other than I'm not very thirsty? What is the other reason for a bicyclist getting hit by a taxi other than that the driver was speeding and went through a red? I'm not saying that Leibniz's theory is wrong or that anyone else that is religious and believes in a higher power is incorrect, I simply am just trying to make sense of his approach in a way that I can understand.