Personally, I think TheLady overdramatizes the etymology of this phrase. As has been pointed out in other nodes, there's a difference in meaning between "love" and being "in love".
"Love" is comparatively easy. I love my parents, I love my siblings, I love my friends, I love my fellow man, I love the planet. Love, whether used to mean the emotion or the action, is something that can be shared with anybody.
Being "in love", however, always means the romantic sense of love. It's not necessarily exclusive or permanent, at least not in this day and age. But it does imply a depth of feeling, concern, and willful sacrifice.
Compare to the Greek words for love.