Friend
Sam didn't exactly have a whole lot of friends when he was growing up, sure he had Miles, but he was kind of an idiot. So Sam had gotten used to going at it alone, believing he didn't need anyone.
He never expected that to change, until he bought a car that was really a talking robot from outer space, a robot hell-bent on keeping him safe.
Suddenly, he had lots of friends, Bee, Optimus, all of the other Bots, Nest, Keala, plus all of the world leaders who wanted to thank him for saving the world, multiple times.
It was through them that Sam finally learned how important friendship was, and it was a lesson he never forgot.