Three times in this psalm God's love is described as "unfailing" V.5, v.18 and v.22. Things of earth may fail or not turn out as we wish but God's love is fail proof, something we can totally rely on. Our church use to sing a song called, "Unfailing Love" (I think it is from Hillsong).
A lot of the motivation for helping the poor comes from two stories Jesus told, one being the Good Samaritan and the other being the story about the sheep and the goats. In context neither of these stories are actually about helping the non-believing poor.Jesus told the story of the Good Samaritan in order to