I-Messages are more effective in influencing a child to modify behavior that is unacceptable to the parent as well as healthier for the child and the parent-child relationship.
The I-Message is much less apt to provoke resistance and rebellion. To communicate to a child honestly the effect of her behavior on you is far less threatening than to suggest that there is something bad about her because she engaged in that behavior. Think of the significant difference in a child's reaction to these two messages, sent by a parent after a child kicks her in the shins:
"Ouch! That really hurt me--I don't like to be kicked."
"That's being a very bad girl. Don't you ever kick anybody like that!"
The first message only tells the child how her kick made you feel, a fact with which she can hardly argue. The second tells the child that she was "bad" and warns her not to do it again, both of which she can argue against and probably resist strongly.
I-Messages are also infinitely more effective because they place responsibility within the child for modifying her behavior. "Ouch! that really hurt me" and "I don't like to be kicked" tells the child how you feel, yet leave her to be responsible for doing something about it.
Consequently, I-Messages help a child grow, help her learn to assume responsibility for her own behavior. An I-Message tells a child that you are leaving the responsibility with her, trusting her to handle the situation constructively, trusting her to respect your needs, giving her a chance to start behaving constructively.
Because I-Messages are honest, they tend to influence children to send similar honest messages whenever they have a feeling. I-Messages from one person in a relationship promote I-Messages from the other.*
*Excerpt from Dr. Thomas Gordon's P.E.T. book