What was Big Willy Shakes going for?
An impassioned speech always seems to do the trick when you're trying to convince someone to commit huge crimes like murder or treason. This is exactly what Cassius does when he and Brutus contemplate removing Caesar from power. Although Cassius claims that men are "masters of their fates" as a way to motivate the conspirators to action against Caesar, there's a lot of evidence to suggest he's wrong.
The play is full of omens and prophesies that come true, which undermines the sense that characters can exercise free will and shape the outcomes of their lives. We should also keep in mind that Julius Caesar dramatizes historical events that have, by definition, already happened. As characters struggle with questions of fate versus free will, the audience already knows what their futures hold. This tends to create a lot of dramatic irony if we think about it.
When we get down to it, we're not sure that Cassius even believes what he's saying. It might just all be part of the act to convince Brutus to team up. After all, the guy does brag to the audience. Cassius compares himself to a metalworker as he suggests that even the noblest of men can be manipulated, or bent, to his will. How does Cassius coax Brutus? First he slyly suggests that the Roman people want Brutus to lead them, then he sends Brutus some forged letters urging him to take down Caesar. It looks like he's trying to control his fate but not succeeding.