And he repeatedly reassures the reader that responding with good middot to difficult people reaps loads of Heavenly Reward.
As he writes in the chapter entitled Peace/Shalom:
The Sages spoke only of one whom the controversy pursues him.
And he flees from it and the peace flees from him and he runs after it [i.e the peace]—specifically this is what's called a "rodef shalom—a pursuer of peace" -- and his deeds will be praised at the gates.
No need to race off in hot pursuit of peace with lovely people!
Instead, being a rodef shalom is davka with people who are difficult. That's the mitzvah. You need to run after peace when peace starts acting like an escaped fugitive.
Note: Needless to say, this does not apply to terrorists and the like of whom the Gemara says, "He who comes to kill you, rise early and kill him [first]."
He also explains throughout the book that if it is impossible to maintain civility around a certain person, then best to avoid that person.
That's right -- peace doesn't always mean being all lovey-dovey. It depends on the dynamics of a situation. Sometimes peace means to embrace the other and sometimes it means to avoid him or her.
Either way, the challenge of difficult people is implied in the actual mitzvah.
You're not a rodef shalom when it comes easy.
You're only a rodef shalom when the shalom is hard to come by.
May we all succeed in living in peace with our brothers and sisters.