Destiny is not a matter of chance, it is a matter of choice.

Any event is indeed caused by some kind of action, any action is an act of will, and will is merely a choice. But is it possible to say with absolute certainty that any specific event will occur in the future? No, it is not. Maybe we would be able to say destiny is a matter of choice if any choice we made, therefore any action we took, and therefore any event that occurred was entirely in our control. We cannot, though, because unfortunately in our world there are external factors that could potentially impede the actions which we decide to take. To assert what you have asserted is to assert that any thing we decide to do within the realm of physical possibilities is not only possible, not only probable, but guaranteed. That is not realistic. Even if one legitimately devotes their life to tend to the "destiny" they wish to fulfill, there is always the chance and indeed the probability that something will eventually get in their way. That is not to sound pessimistic, it is the truth - and it is certainly not to say that one should give up on life, all they want to accomplish, and not bother trying. People do generally accomplish the things that they seriously put effort into. But - there is a chance that I should want to talk to my friends house tomorrow and get struck by a car as I cross the block - that would not be a matter of choice. The very course of events that I have described will most likely happen to somebody in the world in the not so distant future. Their destiny will not be something they chose. Destiny would only fully be in our control if one had the world to himself, without anyone to get in their way. Perhaps there is a student in a Science class as we speak with a teacher that he loves, respects, and admires. That could potentially lead the child to become more interested in the field of Science which could in turn lead him to get a job related to science, all because of the teacher that sparked his interest in it. The boy could go on to make revolutionary discoveries and with them millions of dollars. Or, perhaps the same student is in a Science class with a teacher he hates, has no respect for, and finds extremely boring. The same student that could have went on to become a multi-millionaire scientist could be "turned off" of the subject of science by his mean, boring teacher, which could in turn lead him to get a job related to something completely unrelated to science, which could in turn stop him from making the revolutionary discoveries and millions of dollars that were made, in essence, because of the nice, interesting, admirable science teacher. One comparatively small effect on another persons behalf, completely out of ones control, can have a momentous effect on their destiny. Another example is a student who worked hard to pass a course. He has to take a test in order to pass it. He has taken the action, an act of will, which is merely a choice, in order to cause the event. But the teacher gives him a test that proves too difficult for the student to pass. Was his destiny a matter of choice? What you imply by saying that destiny is fully within our control is that we can control every aspect of our lives. I'm sure everyone would agree that if they could each individually control their own lives things would be better. One might even say it is a good thing that destiny is not within our control, because if it was, we would have no goals to overcome and life would become rather tedious. Life would become boring and maybe even pointless if our destiny was fully a matter of choice. There is always, for anything that happens in this universe, the slightest chance that the biggest effort will be ruined by the smallest coincidence. Maybe that is what keeps things interesting for us humans here on planet Earth. To conclude, to say that destiny is within our control is sweet and encouraging but naive and fallacious. The definition of destiny itself (that you have provided), "a predetermined course of events often held to be an irresistible power or agency" says that there are events that no matter what one does or how hard he tries to change them, the people they involve will ultimately succumb to their destiny regardless.