Carter says Seinfeld took Leno's side because he felt "[Conan's] youth movement thing was against the old time comics." He summarizes Seinfeld's view this way:
You don't take the #1 guy off the air. You don't do it. I took myself off the air. That's one thing. You don't go to a guy who is #1 and tell him: "That's it. You're off the air." Who does that?
We're in show business. This is what we do. We're supposed to look for the best jobs. We're supposed to get them. We're supposed to pursue them. It's not right for Jay to walk off the stage and say, "It's all yours Conan, take it." You just don't do that in show business.
Here's the Google Books excerpt where Seinfeld gives his .02. Basically, he feels The Tonight Show is a meaningless concept since people never say "I'm doing The Tonight Show" and instead say "I'm doing Jay; I'm doing Dave; I'm doing Conan."
Carter ends the section by describing Seinfeld's frame of reference this way: "Jews do not celebrate martyrdom."