My ears hear a significant difference between I love you and I love you too. Do you hear it, also?
If all you ever hear is I love you too, it would seem there's reason to doubt the strength and validity of the love that statement is supposedly expressing. Maybe that's a false assumption, but I feel like if it's only said as a response to your I love you, and it's never just said, then it doesn't hold much weight.
We all want to be loved with no strings attached. We long to hear that love expressed not as a knee-jerk reaction, but as a declaration of reckless abandonment.
It reminds me of my favorite kind of kiss---when Niel kisses me on my forehead or on the top of my head. It seems the most selfless type of kiss, and it makes me feel utterly and completely loved simply because it is neither reactionary nor soliciting a response.
Hearing one sincere, pure-hearted I love you speaks volumes more than a multitude of I love you toos.