Droids didn't dream. When they were shut off it was as if the universe ceased to be until they were turned on again. There were no fantasies drifting through, no memories brought to the front. They were not disoriented trying to figure out reality. They didn't power down again so they could finish or spend the day remembering what happened as they slept. Droids didn't feel either, so C-3PO didn't feel fear when he was powered down. Though there was certainly a possibility that he would never be turned on again, just thrown away or taken apart and he never knowing.
(Things changed when you were turned off. A new universe could be born by the time you awoke. And people. People disappear. People you can't remember knowing.)
.
Droids were things, objects. They could be bought and sold, traded and given away. They could be mindwiped by their new owner in insure loyalty, or permitted to keep their experiences. C-3PO was horrified when Master Luke gave him to Jabba the Hutt. Still, this would not be the first time he changed owners and he was something like ashamed that R2-D2 was so much more accepting. He did his best for his new master, yet could not stop himself from calling out when Master Luke appeared.
(I built you for a purpose. You're a gift. You are what my son left me. I cover you in gold because you're my husband's creation. You alone will come with me when I go to him. You are worthy.)
.
People could change. Droids could change too, but differently. People changed because they chose to. They could make an effort to be nicer or work harder, or listen more. Droids could compile information, come to conclusions based on the facts, and modify behavior accordingly. Droids could be changed by others. They could be opened up, taken apart, and put together differently. He thought it was fear he had for that very thing on Bespin – that Chiewie might have listened to Captain Solo's complaints and would change him. As it was C-3PO had the vague idea he might have marched with a gun once.
(After Bespin he found Master Luke very different indeed. He stated Vader changed him. C-3PO thought that was rather evident (he had a new hand after all), but that did not account for his emotional state. Master Luke said he was changed in another way, with words. Humans were so very hard to understand.)
.
Humans dreamed. They would wake up trembling or smiling or crying or angry. Droids always woke up the same. A blink of lights, systems loaded, and they were ready. Humans had nightmares and good dreams; nonsense dreams and dreams that meant something. Before Bespin, after the Death Star, Mistress Leia wept often, Master Luke was sometimes troubled, but often hopeful, and Captain Solo slept lightly but woke content. After Bespin, before the Second Death Star, Master Luke awoke crying nearly every night, Mistress Leia rose angry, and Captain Solo was gone. After the Second Death Star Captain Solo woke in cold sweat, Mistress Leia was troubled, and Master Luke was content. Humans revealed their lives in their sleep. He found it very strange.
But then, droids did not dream.
(So why did he see a blond-haired boy in that moment before he became aware, just after he was turned on? And why did the boy's disappearance leave him… empty?)