Answer:
The pain-pleasure relationship is simple.
Business process restructuring is another word for change.
Change can be difficult to implement, much more difficult or painful to accept.
Given that organisations are mostly people, and people can, for a myriad of reasons, fight change, the process, therefore, is arduous.
Business Process Re-engineering sometimes means people will become redundant because a particular value-adding process has become outdated by new technology.
It's easy to say "people can prevent themselves from becoming redundant by coming up to speed with new technology". However, what happens to a 50-year-old who still drives a taxi for a living, if tomorrow the market develops a strong affinity for driverless cars? Will the 50-year-old go on to learn how to fix driverless cars at that age? Even if the technology was opensource and non-proprietary, will it not be too late for the taxi driver to enrol for programming lessons or advanced auto mechanics maintenance courses?
Business Processes can be like that. The goal of reengineering the process of an organisation is to achieve a higher level of effectiveness, efficiency, productivity and profitability.
To maximize the pleasure which comes with having a more productive workforce at a lower cost, business owners can (where possible,) give adequate time for changes to be communicated, accepted, bought-into and prepared for.
Cheers!