As it's been said before: droids are slaves. C-3PO is too cheerful about the experience, but R2-D2 isn't. He lets the biological characters know what he thinks about them all the time. They just can't understand him, which is probably a good thing. R2 takes all the risks, does all the work, and gets little of the credit. It's no wonder that he eventually snaps.
-via The Mary Sue