My ex said it to me first, 3 months into our relationship, and I didn't say it back. We were lying down together and she said it, I just kind of froze and didn't know what to do. It turns out, that in the next two or three days after that, when I didn't see her, she spent the whole time crying because she thought I didn't love her. My best friend, who also used to be a good friend of hers, ended up calling me and telling me what a dumbass I was since I loved her and needed to tell her ASAP. So I did.