When did you realize you were falling out of love.

I personally don't think I have ever fallen out of love with someone when I was in a relationship but I saw someone fall out of love with me. We were so happy at the beginning and he was obsessed with me and I with him. Our only problem was the distance and we would stay up to 2 months then maybe meet for a week. After one year he really stopped putting the effort came to find out he met someone else but my story is not the main focus. For those who have fallen out of love what happened what caused it and how far into the relationship did you notice.