The Gospel according to Luke is sometimes called "The Gospel of Mercy," because Luke highlights the actions and words of Jesus that show love for the poor and the outsiders and the marginal.
It is only in Luke's Gospel that we see Mary and Joseph homeless and our Savior laid in the feeding trough of animals. Only Luke has the first announcement of the Messiah's birth made to poor shepherds, a group who lived on the margins of their society.
It is only Luke who gives us Jesus' parable of the father whose unconditional love makes his prodigal son welcome home. And the parable of the Good Samaritan who showed mercy to an enemy.
It is only in Luke that we meet the despised tax collector Zachaeus and see Jesus love him and stay in his home.
Luke is the only Gospel writer who tells us that, even while himself suffering on the cross, Jesus showed mercy to a criminal, crucified beside him.