Fear
A lot of people marveled how fearless Samuel James Witwicky was. What they didn't realize was how not fearless he actually was.
Quite a number of things scared Sam, Decepticons, the idea of losing the ones he loved, failing the Autobots, spiders.
The trick was not allowing anyone to see his fear, because Sam knew that if his enemies, no matter what species, believed he was unafraid to face them, than he would have already won half of the battle.
So Sam would shove all feelings of fear down and always step forward, always keep going, no matter how hopeless the situation might seem, and only those that really, truly knew him would be able to see the truth.
He had gotten out of more situations than he could count with that tactic, circumstances better soldiers had not.
It really was no wonder everyone believed he had no fear. Still, that didn't stop him from freaking out when Skids dropped a tarantula on his head.