Love you is really the lazier way to say it. Saying I love you in full, means much more as love you is usually used toward pets or siblings. While I love you is for people you have a deep passion for, wether it be parents or your true love. When people say love you, you can notice a change in their voice; More drowsy and careless. Of course, someone could just have a habit of saying it that way. Just because someone says "love you" doesent mean they love you any less than saying "I love you". Some people might not even consider them to be different.
