1."I'm always really emotional and feel like I just can't keep my feelings in check."

Contrary to popular belief, brain development does not end when we are very young.  While certain parts of the brain are fully developed early, other parts continue to grow for many years following.  Right before puberty sets in, the brain begins to undergo some major growth once again.  This growth continues throughout much of adolescence.  

The portion of the brain that experiences one of the most dramatic changes is the frontal cortex, and specifically the prefrontal cortex.  This portion of the brain is considered the "'CEO' of the brain, and regulates planning, working memory, organization, and modulating mood..as well as control over impulses and making judgments."  Research shows that this portion of the brain increases in development around the beginning of puberty (on average 11-14 in girls and 12-15 in boys) and continues to develop until the late teenage years. (Society for Neuroscience. 1995 Lydia Kibiuk)  In contrast, the portion of the brain that regulates emotion, namely the limbic system, has already reached its fully developed state (or is very close).

There are two main results stemming from this.  The first is an overload of emotions that cannot be easily reasoned by the developing prefrontal cortex.  As a result of the full development of the limbic system (emotion regulation center), the average teenager has the full emotional capacity of an adult, but has an underdeveloped system for controlling it (prefrontal cortex).
Additionally, it makes teenagers more likely to act on their impulses.  With strong emotions comes impulses to make certain decisions, and with an underdeveloped prefrontal cortex, it is harder for the average teenager to think through the impulses.

2. "My parents just don't understand me!"

Teenagers and adults have different brains, and use them differently.  The biggest differences involve the regions involved in judgment and insight, as well as where we process emotion.  As a result, adults and teenagers tend to interpret facial expressions and tone of voice differently.

In a recent study, Deborah Yurgelun-Todd examined the differences between the brains of adults and teens.  "Todd put teenage and adult volunteers through a MRI and monitored how their brains responded to a series of pictures. The volunteers were asked to discern the emotion a series of faces... All the adults identified the emotion as fear, but many of the teenagers saw something different, such as shock or anger. When she examined their brain scans, Todd found that the teenagers were using a different part of their brain when reading the images."

As a result of this, adults and teenagers may see differntly due simply to a brain difference.  Todd hypothesized that when confronted with "emotional information, the teenager's brain may be responding with more of a gut reaction than an executive or more thinking kind of response" (Frontline Interview.  "Inside the Teenage Brain").  

3. "I just don't understand girls!/ I just don't understand boys!"

Guess what?  Males and females have different brain structures as well.  One of the biggest differences is the way the brain handles communication.  While centers for communication are located on the left side of the brain in both sexes, women tend to engage more of the brain when communicating.

This difference may be attributed to a thicker Corpus Callusum in women.  This thick bundle of fibers allows the left brain to communicate with the right side.  As a result, women use more emotional speech, and are able to speak about their emotions more easily.  Conversely, men speak more in terms of fact and reality, with less draw on their emotion.

Additionally, women have the ability to pick up on emotion in speech more easily, whereas men tend to not notice small emotional ques.

This difference can be seen in several studies involving stroke victims.  Researchers have found that male stroke victims whose left brain has been affected lose language skills twice as often and almost twice as severely as females who suffer from strokes.  Through MRI's, it can be seen that women begin to use their right brain more to communicate following a stroke, whereas men do not.

(Yes, I am a nerd.)