Poor design, consider the things we enjoy and how incredibly arbitrary they are, eating and fornication in particular are quite mechanical, these no reason why AIs couldn't be designed to enjoy their tasks as we enjoy the necessities our biological imperatives.
Although interestingly the enjoyment of classical music may be universal, if you pay attention to it music is really just patterns, upon patterns, upon patterns, I'm guessing we get a little euphoria every time we recognise a new pattern, which accumulates as the patterns do.
In my factory there's classical music playing and the workers all mindlessly blissful.
Which reminds me I just started playing Metal Gear Solid Peace Walker and in one of the cutscenes a drone flys by, singing to itself in a voice very much like Hatsune Miku's which was creepy as hell, specifically the idea that this cheerful little drone loaded with high tech weaponry is firstly intelligent, and secondly, bored.