I don't know about this quote specificly, but Darwin spent almost 10 years working on barnacles to establish himself and came to hate them. It was supposed to be a quick detour before releasing 'On the Origin of Species'. During that time he made a lot of similar statements.
At the time barnacles represented a major taxonomic mystery. They have a bizarre lifecycle that baffled scientists and figuring out what they were related to was baffling
I assumed it was something more inconsequential and personal. It's the sort of thing I usually say after doing something particularly embarrassing that I can't get out of my head...