Rav Nachman of Breslov - 7

Sichot HaRan - 80
"People have the strength to weaken the resolve of another person, distancing the truly righteous from worshipping G-d. The strength is greater than the evil inclination's strength. The evil inclination's strength is limited to the level the person is on. Man, on the other hand, is more general, for he includes all the worlds (is not as limited)."

The evil inclination often works hard to harm us spiritually. However, the greater harm comes from other people. Peer pressure is one of the hardest things to combat because people like to feel part of a group, so often they will follow the crowd whether it is for the person's good or not. While I grant that we shouldn't always be leaders, we need to, at the very least, be in control of ourselves. We need to remember that we have the ability to make our own decisions and should not simply follow the whims of another.