My character goes down, I'm cheerleading my companions to avenge me. I'm not checking out or moping. I might be angry, but I'll focus that at the monster/trap/villain that did it, or myself for being stupid, not take it out on the DM or other players (unless some fucker didn't heal me when he should have).
This is a good approach and I endorse it, in principle. Still, I think it's worth pointing out that in practice this can be harder to do than it might seem, for many people.
Depending on how long it takes a player to create a new character and for the game to find a plausible in-setting "re-entry point" for him, character death can mean you're out of the game for hours, possibly even the rest of a session; it's hard for a lot of people
not to "check out" at that point for simple lack of being able to contribute, even if they know better than to let themselves get into an evening-ruining bad mood over it. And on a gut level, every player knows that the monster/trap/villain which killed their character
is the DM, in the end, so that's where our limbic systems naturally want to direct our displeasure.
Emotional maturity is the ability to put aside these reactions as needed, of course, and it should always be recommended. I just think it's a good idea to design games with at least an optional mode for groups where it's, shall we say, not as developed as might be preferred.