When one is met with the sentence “I love you”, the proper response (if applicable) is “I love you too”. I, of course, don’t reply this. I reply either “And I love you” or “I love you” with an emphasis on “you”.
So, why do I do that? Because I find the “too” confusing. A proper use of “I love you too” would instead be “I love the smell of grass in the morning… I love you too” (caricatured, of course). Adding “too” means you’re chaining together, so this would be proper use.