Because R2-D2 is so iconic as Luke’s Droid™, and even in Clone Wars and RotS as Anakin’s Droid™ that I think it’s easy to forget that Artoo is Padmé’s.
I mean the little guy was cheeky and confident and saving his mistress’ life before she even met him. And that’s not to say he doesn’t bond with Anakin right away in Phantom Menace, but the thing is he stays with Padmé
for the next…ten years? Eight years at absolute minimum.
And the thing is Artoo was assigned to the royal cruiser, almost certainly property of the Crown–by rights, Jamillia should own him by this point. And yet there he is with Senator Amidala. Did he just decide to stay with her and no one questioned it? Did she buy him? Was he a gift? No matter how it happened, by the time they run into Anakin again Artoo and Padmé are almost literally inseparable.
Artoo stays with her when she’s posing as a fighter pilot. Artoo–an astromech droid, not a specialized security unit–is the one who guards her while she sleeps.
Artoo accompanies her when she’s undercover, for no apparent reason. Why do you need an R2 unit at Varykino? What possible benefit could he be? If anything he draws unwanted attention. He’s there because Padmé
wants him there. (And, I suspect, because he refused to stay behind.)
Artoo saves her life. Artoo is at her wedding. Artoo takes care of her husband for her (and yes, despite shipping Padmé with Sabé with the passion of a thousand suns, I think it’s adorable that she and Anakin assign their respective most trusted droids to take care of each other.) Artoo is there for the birth of her children. Artoo watches herdie. And Artoo spends Leia’s entire life as the only person in the universe who knows everything. And he has to carry that knowledge by himself, because Threepio doesn’t remember. And yet over the decade-and-then-some between series, he manages to keep them together. Because they’re married Artoo is Padmé’s, and Threepio is Anakin’s, and they’re supposed to do this together.
Fast forward to ANH and we find Artoo exactly where he’s always been, protecting his mistress. He takes care of the Skywalker kids exactly as fiercely as he ever took care of their mother. (”I found her! She’s here, she’s here, she’s here!” ”Do you think he likes me?”)
He crossed half the galaxy to find Leia, who wasn’t even his owner; because she was Padmé‘s daughter and he had a duty to fulfill. And just as an added bonus he found Luke, too.
And he never once leaves them.
And the fact is it all started because Padmé’s best friend wanted to play a trick on her.
I will forever maintain that the reason Artoo was the droid Leia used was because Artoo went to go find her as soon as the Tantive IV came under attack. (Came under attack by Anakin Vader, no less…)
