Family
"They are your friends?" a politician had once asked him about the Autobots.
"They are, but they're also much more than that," Sam had replied fervently, "They are my family."
Sam never knew that those robots were listening in, and he would never realize how much of an impact those final four words had on them all.
Sam would probably never understand how his love for them, and their subsequent love for him, had put all of their sparks at risk, because he was human, he was fragile, and they truly hoped he never would.
Knowing Sam, he would probably try to do something stupid to stop them from having to feel that pain, he would try to leave, and they could not let that happen.
After all, he was their family too.