We can only wonder why things happen when and how they do. Maybe you are a believer in coincidence, which actually means you aren't really a believer in anything. It could also be that you believe that fate rules your destiny. When you think about it, this basically means that regardless the decisions you make, what happens was already predestined to happen. Fate basically removes all need to even think for yourself. The truth, it seems, is in the middle of these two philosophies. Coincidence cannot be the governing force in our lives because it would preclude the possibility of consequences for our actions. Fate too is equally untenable because fate cannot account for all probabilities as their are hundreds and in many times thousands of choices made each day.
We are the product of choices, big and small. Making the decision to leave ten minutes early may allow me to miss the traffic jam, or it might put me right int he middle of it. The amazing part of life is how the choices we make can turn into pivot points toward great things. The only prerequisite to happiness is realizing that you always have a choice. To paraphrase the English poet W.C. Henley, "We are the masters of our fate. We are the masters of our souls." Good words to live by.