That's an easy one. Destiny is a contradiction. If everything is destiny then what's the point doing anything. Why not just go and hang yourself and get it over with. Destiny takes away every joy and sorrow in life, and just makes everything one long pointless journey to an inevitable goal. My experience is that people who say they believe in destiny really don't. They just use the concept for them to avoid having to think about cause and effect in life.

I'm not so sure about that we have free will either. Our brains are wired to react rapidly and powerfully to percieved dangers a lot more than what would be rational. Is that really free will?