Any deed done with reward in mind creates an expectation which, if not met, causes suffering. Similarly, you will end up focusing more on the reward than the task itself and will unable to be loyal to the task. The only reason you must do a task is because you must do the task i.e. it's your duty to do so (either self-imposed or originating from a construct you follow).
This is such a conflicting piece of advice that goes against the fundamental reward-driven human biochemistry. But I've found that in following it, you really begin to tune out suffering and start to discover something that's a bit above being human.