Hm, I see your point... It's kind of funny. DR1 cast were friends before being wiped, but took little time to mourn when the killing started.
DR2 cast were all Despairs before being wiped, and really had to struggle to keep going once the killings began.
Very interesting.
I really think "Byakua" was the one who set the tone in both games.
In DR:1 he's the intelligent guy that basically tells everyone they're on their own, screw everyone else. Makoto gets betrayed in the first chapter, so even he gets a bit wary of trusting too much.
In DR:2 he's the intelligent guy(That's a fake, but details) that tells everyone to work together, and that no killings would happen on his watch, and sacrifices himself before letting anyone else become a victim. This makes the goal for the cast to carry on his will and try to get along with one another. Oddly enough, this is Nagito's goal, in a way.