Commenter Fitzmac on Is hysteria part of the liberal psyche?:
The hysteria of the Left is their way of responding to their inner doubts that their program cannot succeed. Because they are incapable of silencing their doubts in an internal manner, they have to do it demonstratively, make a show of it, which lets them gain reassurance because they see other people doing the same thing and so can persuade themselves for a short time that their beliefs do in fact correspond to reality.
It is a matter of duplicity, but also, more fundamentally, of mental illness.
That makes perfect sense -- a large part of the reason the Left always chants at their demonstrations is to reassure themselves; it's a strength-in-numbers kind of thing.
Imagine how good it must feel to be at a demonstration and see lots of other young people like you, all screaming and raving and ranting and chanting. It must fill you with a sense of belonging, a sense of righteous fury.
It's also hard not to get the sense that the Left's histrionics are a kind of acting that isn't entirely dissimilar to what sociopaths do: it's a way of trying to convince others that they must in fact be right, otherwise why would they feel so passionately?
Think in terms of Bill Clinton wagging his finger admonishingly and saying, "I did not have sex with that woman!" This is part of a sociopath's box of tricks: say something angrily enough and people will be convinced that you're telling the truth -- after all, why else would you be so angry?
There's another group the Leftist demonstrators may have even more in common with though: toddlers. When you're three, you don't have a lot of self-control, so when you're frustrated, all you can do is throw a tantrum.
Sadly, some of those toddlers never grow up.