When you're training in a big group, the energy (both
positive and negative) of the group is contagious. The right combination
of people, combined with the enthusiasm of the leader, can lead to
excellent workouts for many of the people in the room.
Likewise, it only
takes a few people complaining or slacking to drag the energy level of
the entire room down. Gravitate to people who push hard regardless
(Durst, Gaja, Carson, etc), and be aware of the energy that you are
contributing to the group, because it does affect those around you.