Friend, enemy, and stranger are labels invented by the ego. A friend is necessarily a person who helps me.... An enemy is someone who harms me, and a stranger is a person who neither harms nor helps me. Consequently, we feel attachment for the friend, aversion for the enemy, and indifference toward the stranger. We are blinded by these views.
Until we can go beyond these deluded interpretations and come to see our enemies, friends, and strangers as equal, our hearts can't grow genuine love and compassion. As Lama Zopa Rinpoche says, the love we feel now (for our friends) is indeed love, but it's unstable because of being based on attachment. There are strings attached: as long as you help me, of course I will love you, that is to say, want you to be happy.
We need to argue with our ego's views, gradually seeing the illogic of them, discovering that in actual fact our friends, enemies, and strangers, from their point of view, are completely equal in wanting to be happy.
This equanimity is the stable foundation on which to build genuine love, compassion, great compassion, and bodhichitta.