The loss of something many adults hold as precious. The point before they learned what it was like to be living in a less than perfect world.
No longer seeing the world through the wide eyed trust and acceptance of a child. Usually is accompanied by an encounter which leaves you wondering what there is to believe in. Often when you realize your parents ( or some adult ) is not all you had built them up in your mind to be.
Some attribute the first sexual encounter to be the point at which a person loses their innocence. Loss of trust in other ways can evoke a condition which causes one to be more guarded with their feelings and trust. Being lied to by someone you love and finding out can cause it.