I feel an obligation to the story to keep going. I have been frustrated with so many great stories on this site that have never been completed. I feel strongly that I do not want to do that to others. I do not want to leave them with the plot hanging over their heads or the relationship they have developed with the characters broken without explanation. But it is the sense of responsibility to the story itself that really propels my writing. I need to complete the thought because it is mine. Like tending a plant in a garden, I do not want it to die before its purpose is fulfilled.
Comments from the readers makes the completion process easier. Please comment to any writer that you are following. It really does help.