"... because you loved me before the creation of the world." John 17.24
Altruism refers to the noble sentiment of serving others with no thought of one's own advantage. It is the very opposite of the desperate drive for self-gratification which powers contemporary society and is encapsulated in the word "narcissism" (derived from the ancient Greek myth of Narcissus, the boy who fell in love with his own reflection - and suffered the consequences!)
Altruism describes the love of God, both within the Trinity and towards creation. Father, Son and Holy Spirit exist in a relationship of mutual adoration and service, while for the love of creation, God set aside his glory and devoted his life in order to redeem the world from evil. The whole Bible bears witness to God's altruistic gesture, which impacts on us individually.
Even before we were born, God loved us. Where we fail in our obligations to him, as our creator, and towards one another, as neighbours, God takes the blame so we can go free, standing tall in his righteousness, not being judged as our sins deserve. Even when our attempts to express our gratitude fall short, the Holy Spirit intervenes to pray for us and Jesus himself takes those prayers to the Father, as our heavenly advocate.
So we have no need to justify our existence. Not only would our attempts be futile but it's not necessary because God has done it for us. It means we are free to love God and one another, without any thought of turning it to our advantage. We too can love - and live - altruistically. The world does not understand this: "There must be something in it for you - even if it's just a good feeling..." No! Christians understand that we have nothing to prove: our worth is in who we are, not what we do.
Of course we crave to love and be loved by our fellow creatures: that is what creates community and binds together families. But let that love be like God's: without strings attached. A love which demands something in return is false, true love is unconditional because that is the way God loves and God is love (1 John 4.16)