'I' statements were invented by Thomas Gordon in the 60's as a way to help children communicate with their parents. The technique is so useful it's used most often in couples therapy, and could (and should) be used in any relationship in which it's important to convey your feelings.
The Problem With You Statements
The oppossite of 'I' Statements contain the word 'you' more often. Such statements, more often than not sound like accusations and blame. In doing so they often carry some shades of judgment which closes down communication lines. It puts the person on the defense, making him unable and unwilling to be open to what you have to say and truly listen.
When working with adolescents, I've noticed that 'you statements' will often enhance the negativity and escalate the tension.