Strangers are our friends and families too... in many of our previous lifetimes if we believe what Buddha says...
Lord Buddha said, “In previous lives, I have killed all of you before and you have all slaughtered me. Why should we now be attached to each other?”
In Lamrim, we learn that we need to practice equanimity and should discard the thinking of categorized others into the 3 groups of : friends, enemies and strangers. This concepts are created by our deluded mind.
This all started because we discriminate between ourselves i.e. the "I" and "others". Due to the influence of the Three Poisons, we continue to discriminate and creating more sufferings to ourselves.
Equilibrium meditation by Lama Zopa:
For countless rebirths I have been discriminating other beings as friend, enemy or stranger with the self-I consciousness. Chandrakirti said, “Where there is self-I consciousness, there is discrimination of other.” From discriminated partisanship between self and other, attachment and hatred arise.
All misfortune arises from acting under the influence of these negative minds.
The self-I consciousness causes attachment to self, which produces attachment to my own happiness.
The entire range of negative minds arises from the above.
Anger is caused by greed and self-attachment and makes me discriminate against whoever disturbs my happiness, producing the enemy.
Attachment creates the friend, who helps, and determines the enemy, who hinders.
Ignorance labels those who neither help nor hinder as strangers.
Anger makes me hate and harm the enemy; attachment makes me cling to and help the friend; and ignorance makes me see the stranger as having a permanent self-nature. By acting under the influence of these negative minds, I lead myself into difficult and suffering situations.
Attachment creates danger and suffering for myself and others. The whole earth is in danger of exploding. Attachment offers no peace and brings only suffering.
So it is very important that we practice equanimity to check our attachment to friends, indifference to strangers and hatred to enemies. We must try our best not to discriminate.