"You told me you loved me, then." Tell me: Why? What is the difference between NOW and THEN??

FYI: Only 10% of posts might be about the real me. But I've always wondered...what happens between now and then? This is about changes in feeling in everyone's life...this could be about anything... You could think fondly about some experience one year and the next, your feelings could be indifferent. What can account for that? Or is it chemical. The change from THEN to NOW. Mystifying. What do you think.