Adams has said that Marvin was just a “one-off” gag that became so wildly popular he had no choice but to continue him through the series. IIRC, it was Zaphod who refers to Marvin as paranoid. Given that Zaphod was more of a “style over substance” kind of guy, it seems natural that he’d go for the rhyme and not the reason, as it were. 
RIP Douglas Adams. We miss you, man, where ever you are.
Marvin is still my favorite sidekick paranoid android. The reaction of the crew after they are sick of Marvin’s complaining adds great hilarity to situations. You hear this android and think, who the hell programed this thing, and why didn’t they wipe it and start over.
Marvin was a prototype built by the Sirius Cybernetics Corporation, whose fundamental design flaws were completely hidden by their superficial design flaws! With their mile-high slogan “SHARE AND ENJOY” that was so heavy it sunk halfway into the planet, the top halves of the letters then reading “GO STICK YOUR HEAD IN A PIG” in the local dialect.
Share and enjoy,
Share and enjoy,
Journey through life
With a plastic boy
Or girl by your side
…and when it breaks down
Or starts to annoy,
Like eats your hat
Or has sex with your cat
…Just bring it to us,
We won’t give a fig,
We’ll tell you
“Go stick your head in a pig”.
Mr Adams was a genius.
They were the first ones against the wall when the Revolution came! 
Obligatory obsessive HG2G fan nitpick:
Their marketing department was the first against the wall when the Revolution came.
(Wasn’t it?)
Zaphod also liked alliteration. He referred to Arthur as “monkey man,” although he clearly was an ape.
“Paranoid Android” was not part of Marvin’s name, just one of many insults. It just happens to have stuck.
Wasn’t Zaphod the one who nicknamed him “the Paranoid Android”?
In any case, in a way everyone WAS out to get him. Zaphod used him for menial and dangerous tasks he didn’t feel like doing himself. He was constantly being thrown into space, shot at, made to park cars, flung into the sun, etc, and had every body part replaced (except the bad leg with the pain all down his diodes)…
Life. Don’t talk to me about Life.
Nobody mentioned it.
But somebody would have eventually, so beowulff was merely doing a preemptive strike. Better to spend 10[SUP]-42[/SUP] of one’s brain capacity for a quintillionth of a second now than have to waste 10[SUP]-24[/SUP] for a full picosecond later.
FTR, Marvin does describe himself as, “Manically Depressed”.
For what that’s worth.
Brain the size of a planet and he never seems to be manic for even a second.
You think you’ve got problems? What are you supposed to do if you are a manically depressed robot? No, don’t answer. I’m ten million times more intelligent than you and even I don’t know the answer. It gives me a headache just trying to think down to your level.
Quick Question… My memory is hazy but didn’t Marvin show up after being destroying in Disaster Area’s Sun Dive stunt ship?! How did they explain that or was that the Marvin who’d been waiting in the Magrathean… later Milliways… car park all those Mellenia?
It was that Marvin - there was only one, but he ended up being 6 times older than the known universe. He was rescued (improbably) from crashing into the star Krakrafoon by a scrap metal merchant, who replaced his missing leg with a metal rod, which happened to be the metal wikkit.
Si
In Life, the Universe and Everything Marvin says he was rescued by a scrap-metal merchant. The merchant gave Marvin a fake leg.
Marvin tells his story to Zem the mattress on Squornshellous Zeta. Then the Krikkit robots come and steal Marvin’s leg and kidnap him.
Correcting my own post…
Marvin was actually 37 times older than the universe itself.
Si
Mr. Adams, being a staunch athiest, would probably explain much funnier than I can, that being worm food is much nicer than eternity in church anyways.