Relying upon a Spiritual Guide, who created this idea? Is it actually a necessity for liberation?

I have a friend who is in/with the New Kadampa Tradition (NKT) and she states the importance of relying upon a spiritual guide. I know that this meditation practice is a part of the Tibetan Lamrim practice. What I ask is who created this "necessity" or concept in Buddhism? This seems to me that it completely contradicts the Buddhas teaching! From the mahaparinibbana sutta : > "Therefore, Ananda, be islands unto yourselves, refuges unto > yourselves, seeking no external refuge; with the Dhamma as your > island, the Dhamma as your refuge, seeking no other refuge. There is more to that sutta but I do not want to link massive paragraphs. While conventionally we need teachers to teach us things. We even had a teacher, our parents, teach us how to bathe correctly! Though from my understanding in Buddhism, the Buddha IS the teacher! He IS the example and the dharma IS his teachings of the truth. So who taught this necessity for a teacher and to what degree is this a necessity? From my understanding only a Buddha is self realised, and arhant is a disciple so by definition an arhant has relied upon a teacher, but that teacher can specifically be Buddha, via the suttas.