I think I can help you answer that question. You see the only one that truly wants help from the traveler is the cabal. You even hear the dialogue in game that says the cabal seems to be running away from something. So it can be implied they just want the travelers protection. (For the taken king video it is implied they are running away from oryx.) the hive are just in it to destroy all that is good. They really don't have a purpose other then to conquer and kill. The fallen are just looking to weaponize the traveler to be the ruling house. (As you can tell by the different houses.) the vex are looking to make everything a machine. They see the traveler as an enemy due to it being a sign of new life. (The traveler was able to present new life to different planets in the solar system. It's even said that the traveler made Mercury something beautiful and the bed destroyed that something and made it into a machine.) the traveler was looking for someone to become codependent with. And, you know, I'm guessing the people had light (good) in them. So when the enemies came knocking at the door many people gave their life for the traveler. (Traveler tried helping in whatever way he could.) and in the last city on earth, population had worn thin. There wasn't enough light to keep the traveler healthy. But he felt in debt, hence he made the ghost. And since he is pro life, he made it so that it can bring back the dead instead of sending the living to their death. Hence the guardians were born to fight of the darkness and protect the last city. [spoiler]some of this is speculated, but I've done my share of research. Hope this helps.[/spoiler]