Reason 1: When you feel it within your soul.
Love is such an existential subject that saying "I Love You" must come from the same place love emanates. A place where existence and presence melt as one within the body. The soul which knows love as home is the most authentic space from which an "I Love You" can be offered.
When it is given from the soul, love presents the greatest sense of freedom and the purest sense of kindness. Put your beloved in your heart and envision yourself saying "I Love You".
BUT ON THE BRIGHT SIDE, if he or she belongs there, then "I Love You" also does.
I see you in my heart.
p/s: I Love You.