> The drudgery and boredom of living in a "perfect" world is a constant theme throughout the stories.

You can of course interpret the novels however you like, but that absolutely wasn't Banks' intention when he wrote the series. See the quotes from other comments.

> In one tale, the entire crew of a spaceship deliberately infect themselves with the common cold just to feel something.

Or they just do it because why not? If you'd never been ill, you'd probably be curious as to what it felt like.

> In another story, people turn off their safeguards and go rafting on a lava stream, causing themselves intense pain and even dying, only so they can finally experience some real excitement.

I think in the story the lava-rafters were having a great time, and they were fairly unusual... and people in our culture risk pain and death doing sport just to feel excitement. In the Culture they just have additional options, such as rafting on lava.

Most of the Culture citizens were happy enough with their exploration, art, travel, genetically-enhanced sex, implanted drug glands, games, sports, and so they never got around to lava rafting.

Besides they have the right to go to sleep and never wake up if they are not happy.