Given the short time of the original, I always get a kick out of Luke's response to the Rebel asking him if he wants a new Astromech. "Not on your life. This little droid and I have been through a lot together" or whatever he says.

At which point my daughter and I turn to each other and say, "Yeah. For all of two days!"

In fact, during those few days, what exactly did they go through together? After buying Artoo, Luke cleans him up resulting in Artoo accidentally projecting a holo-message. Then he runs away and gets Luke jumped by Tuskins. Then they hang out at Ben's place, and wind up in Mos Eisley (where the droids don't accompany Luke and Ben inside the cantina). On the Falcon, Artoo seems to hang out more with Chewie than Luke. Then at the Death Star again Luke goes off without Artoo.

I guess Artoo has charisma.