ElectricZombie,
"Ideally, I would rather dive with a buddy. However, not many people are good divers. There are maybe 5 people that I would feel comfortable diving with." - Johnathan Mitchell NAUI Instructor
Maybe you should train your students to the point where you "would feel comfortable diving with." If you don't feel comfortable diving with them, why do you certify them?
"Underwater hunting, mostly solo."
This is the norm, but very effective hunters often hunt in pairs as a team.
"When you teach a class, and go to do checkouts, you are solo the whole time. "
My students are trained to save me before I take them to the ocean. My dives with students are not solo.
"When you volunteer to assist someone with their checkout dives, you are solo."
I only volunteer to help those who teach a comprehensive class.
"Photography, solo."
Why? I've buddied with photographers many many times and I've had buddies while I've shot pictures.
"When you DM on a charter, you solo dive."
You nailed this one. Keep thinking, you might come up with another legitimate reason for diving solo.
radagalf,
"Why can't they just keep up?"
Ever consider slowing down? Your air will last longer and you'll see more. Diving should be a team effort. Neither diver will do exactly as they please, but both will enjoy the dive and each gets to do some of what they want.