For I have loved him [Abraham], because he commands his children and household after him to observe the way of God (Genesis 18:19).
God knew that Abraham would be able to convey the Divine teachings to future generations, because He knew Abraham to be capable of overcoming his intense love and apply stern discipline when it was needed.
In my work with addicted individuals, one of the most difficult tasks I have is to convince their family members, especially the parents, of the importance of "tough love"; that condoning destructive behavior actually encourages it, and enables it to continue and worsen. Although Abraham loved his son Ishmael, he did not allow these feelings to deter him from the necessary discipline (Genesis 21:9-14).
Love is an admirable feeling, but it can be destructive if it is misdirected. Sometimes we must rein in our love and apply strict measures. While doing so will cause us great distress, our failure to do so will ultimately cause even greater distress to all concerned. Loving parents submit their infants to immunization which may be painful. "Tough love" is not cruelty, but like some life-saving medicines that taste bitter, it is helpful albeit unpleasant.