Does one's will affect the course of their destiny? Forgive me as I may be a very supersitious person at times, but there were many instances when I desired something so much, I didn't get it. And when I barely though of that desire, I obtained it.
A person's willpower very much changes their interactions in life. For example, if you use a "curse" of sorts, but with a good intention, and enough willpower, the effects will not be negative, but actually twofold, helping a specific party, while causing only limited collateral damage.
As far as achieving desires, many would suggest it's only a fluke of the human mind, but it's also possible that the philosophers are right, and "Man wants most what he cannot achieve," and so when you really want something, you don't get it, but if you barely think about something, then you get it.