Humans are by nature very sensitive beings.  We are so emotional because we perceive everything with the emotional body.  The emotional body is like a radio that can be turned to perceive certain frequencies or to react to certain frequencies. 

The normal frequency of humans before domestication is to explore and to enjoy life; we are tuned to love.  

As children, we don't have any definition of love as an abstract concept; we just live love.

In the way we are - the emotional body has a component like an alarm system to let us know when something is wrong.  The emotional body perceives emotions, but not through the eyes.  We perceive emotions through our emotional body.  Children just feel emotions and their reasoning mind doesn't interpret or question them.  This is why children accept certain people and reject other people.  When they don't feel confident around someone, they reject that person because they can feel the emotions that person is projecting.  Children can easily perceive when someone is angry and their alarm system generates a little fear that says, "Stay away!"  They follow their instinct, and they stay away. 

We learn to be emotional according to the emotional energy in our home, and our personal reaction to that energy.  That is why every brother and sister will react differently according to how they learn to defend themselves and adapt to different circumstances.  When our parents are constantly fighting, when there is disharmony, disrespect and lies, we learn the emotional way of being like them.  Even if they tell us not to be that way and not to lie, the emotional energy of our parents, of our entire family, will make us perceive the world in a similar way.  The emotional energy that lives in our home is going to tune our emotional body to that frequency.  The emotional body starts to change its tune, and it is no longer the normal tune of the human being.  We lose our innocence, we lose our freedom, we lose our happiness and we lose our tendency to love.  We are forced to change and we start perceiving another world, another reality. 

Source: The Mastery of Love by Don Miguel Ruiz