The spirit of Hogwarts believes that Harry has suffered enough in his eleven years of life and calls in a protector to guide and care for him. Not slash, rating for violence in later chapters.
Harry is told of a new world which might need his help. There he finds new family and new ways to annoy people. (Not Slash)
Harry, Neville, Clark and the rest have each known Xander Harris as their father, grandfather, father-in-law, etc. Now they want to try and do for him what he did for them, by saving him from a bad home and a life fraught with death and despair. (Final instalment of the Harry Potter: Aspect of Death)