I feel it's a reflection of a deep love shared by two people that started out passionately, pure. A connection that no words can perhaps describe at the time. The two became "one". The warmth, comfort. Pure. Then things, for what ever reason fell apart and that connection was lost. But for one person that love is not something to be easily forgotten! That's why they are strangers and you can't feel them anymore! The "light that was guiding" is the love has been taken away. The person is confused and crushed (does not understand why it's been taken away from them and why the other person has "changed" probably suddenly). Love by numbers, I feel he is saying the love shown, in reflection, seems as though the other person was "painting by" numbers. You are guided and instructed on how to paint! Therefore, the love was "guided" and not pure. But this person can't forget. The beginning. What they thought was so true. But yet they are so hurt by the fact the other was far from everything they believed in. Yet they still crave the beginning.