I always liked what Charles Darwin wrote to J. D. Hooker in 1853:

After describing a set of forms, as distinct species, tearing up my M.S., & making them one species; tearing that up & making them separate, & then making them one again (which has happened to me) I have gnashed my teeth, cursed species, & asked what sin I had committed to be so punished […]

It describes perfectly the feelings of a biologist while doing taxonomy work.