Life, The Universe & Everything, By Douglas Adams

"The robots aren't enjoying it, sir."
"What?''
"The war, sir, it seems to be getting them down. There's a certain world-weariness about them, or perhaps I should say Universe-weariness.''
"Well, that's all right, they're meant to be helping to destroy it.''
"Yes, well they're finding it difficult, sir. They are afflicted with a certain lassitude. They're just finding it hard to get behind the job. They lack oomph.''
"What are you trying to say?''
"Well, I think they're very depressed about something, sir.''
"What on Krikkit are you talking about?''
"Well, in the few skirmishes they've had recently, it seems that they go into battle, raise their weapons to fire and suddenly think, why bother? What, cosmically speaking, is it all about? And they just seem to get a little tired and a little grim.''
"And then what do they do?''
"Er, quadratic equations mostly, sir. Fiendishly difficult ones by all accounts. And then they sulk.''
"Sulk?''
"Yes, sir.''
"Whoever heard of a robot sulking?''
"I don't know, sir.''
"What was that noise?''
It was the noise of Zaphod leaving with his head spinning.