Unfortunately, being a great guy isn't all there is to it.
Few men that come out of boot camp would say that their drill sgt was a great guy, someone they just loved to death, and a guy that made coming to work an immense joy. But, a drill sgt's job isn't to be every recruits' best friend. His job is to rip away egos and forge a team in the most competitive job there is -- losing is literal death.
The NFL is not life and death and I am not comparing football to the military. The point I am making is that successfully coaching football is about building a team, sacrifice, and unwavering focus on the ultimate mission. Crushing young men's dreams of playing in the NFL or, more precisely, playing on an NFL champion team is a heartless and ruthless job, and not something one can be particularly "nice" about.