When I started donning the tefillin some years ago, I had a pamphlet given to me by a Lubavitcher and I used it at times for the Shema. However, there's an addition before the Shema in the pamphlet and not in my siddur. I've been including the addition of "Hareini mekabel...Ve-ahavta lerei'acha kamocha" before saying Shema and haven't questioned it much until now.
Is this a tradition specific to Lubavitch or is it more widespread? What is the purpose of its inclusion?