In the books it is mentioned many times that Harry has to live at the Dursleys' because of the protection charm but was all the hardship he went through during his years of living with them necessary as well? In the HBP, Dumbledore clearly criticizes the Dursleys for how they treated Harry but why didn't he try to modify their memories to really forget about Harry's true identity and maybe love him as a nephew?
If you change a person’s memories, you change a person’s personality. A Harry Potter without those harsh memories of the Dursleys would arguably be a very different one from the one in the stories and possibly not one strong enough to succeed in his future. I understand you wish to change the Dursleys' themselves, but the effect would be the same on Harry as his actual memories would be different. We are all products of our environment, and without overcoming strife there is little encouragement to develop strength.
For further reading on the change of personality due to memory, see any of the studies on Alzheimer's disease. This college student's work is very insightful.