Kryten wants to be human.
And in episode 20, DNA, he gets that wish fullfilled. After finding a machine that rewrites the DNA of living things, Cat accidentally uses it on Kryten, who then becomes human. However, as Lister soon finds out, Kryten really doesn't know that much about how human bodies work!

For starters, Kryten is confused that his eyes don't have a "zoom" feature, nor split screen, nor slow motion, nor Quantel (whatever that means!). He also thinks that human nipples are used to regulate body temperature and get radio stations. He also thinks humans need to recharge like mechanoids do, with a cable plug.
He is also completely disgusted at what the male anatomy looks like. =P And this in particular eventually leads to proof that even though he has become human, Kryten is still a mechanoid at heart...It isn't long before Kryten discovers what happens when something, um, excites human males. Rather concerned, Kryten takes photos of what happened to him in that state and shows them to a rather shocked Lister, who asks what he was looking at when it happened. While one might expect him to reply with some kind of beautiful girl, Kryten tells him that it was a vacuum cleaner in an appliances catalogue! To which Lister says, "You shouldn't be getting erotic thoughts about electrical appliances!" It's obvious that even though Kryten has acquired a human body and all of its functions, it doesn't mean he really is one! Lister tells Kryten that he should just change back into being a mechanoid, since that is what he really is. But Kryten vehemently disagrees, insisting that he could never go back to being a mechanoid, that being human was his dream.

Later on, Kryten is rather rude to his fellow spare heads. Later it sinks in that he was mean to his own kind, and he says as much to Lister. Kryten finally agrees that he should go back to being a mechanoid.

How is this all happens in the book, Last Human, is very different. Instead of the Cat accidentally getting Kryten stuck in the machine which just happens to turn him into a human, Kryten goes to it on his own and makes himself human. Kryten also finds out what it's like to be in pain when he falls down staires, due to the Cat's clumsiness, and the two of them actually get into a fist-fight!