Salvation is a personal decision - the individual person's choice to follow Jesus to resurrected everlasting life.
But the reason why such a choice is made is for the sake of others; because of what we want to do in the Heavenly state-of-being.
This doing entails love of others - including God, and 'fellow Men' - and becoming more-divine - that is becoming fully aligned with God's creation and the sub-creation of our fellow resurrected Men.
(The term for this becoming fully aligned with God's creative purposes is Theosis.)
Thus; while Salvation is individual, Theosis is relational.
Therefore; the reason-for Salvation is Theosis.