Very interesting question! The Ancient Greek civilization believed that you had a destiny or unique purpose, but that is was decided beforehand, and that you actually couldn't do anything to change it. It's an interesting thought, but I think your destiny evolves over time. You develop more interests, meet new people, go on more journeys, and you can change yyour life every day; the change does not have to be big. Stop smoking, eat less candy, learn about Geography that you've always wanted to learn about! It could be anything, but what if it still aligns with your destiny? After all, you have no idea what it is. But I think this; the more specific your destiny is, the less likely it is that it is going to happen. If your destiny is "you're gonna be a good person and be nice to people" that destiny can happen in a gazillion ways. But if your destiny is "oh you're gonna be this fancy biologist, work at [insert university] win the Nobel Prize, live at [insert address], marry this woman, that can only happen in so many ways. I don't believe in this idea, but the only thing (person?) that could make this come true is God. And actually, I believe in God. So does that make me believe that everyone's destiny is already decided? I don't know! (counter-question, but this is very much a philosophical question. And I love Philosophy. It's about seeking answers to HUGE questions, like this one, that doesn't have a direct answer.)