Instinct.  There are certain behaviors you see in animals that defy explanation.  How do they know to do that?  It is inbred.  Did you know that loving is the same?  If you are not feeling the desire to reach out to another when they are hurting, you have shut off your natural behavior.  Perhaps this has occurred from being hurt at some point.  You can undo this.  You are safe now.  You are loved.  You are worthy of being loved and always have been.  As you come to remember these truths, your instinctual desire to love others will come back online.  Reach in before you reach out and remind yourself you are treasured beyond measure.