So, I'm reading the 1989 interview that the Lucasfilm Fan club had with Lucas, and I noticed one of his answers were quite interesting. Spoilers to Episode 3 to follow.

Lucas's 1989 Interview



*Spoilers*





If you've read the Episode 3 novel, you read a more in-depth answer to why Obiwan and Yoda were able to become Force Ghosts. Qui-Gon tells Yoda that the secret to immortality is complete selflessness and compassion, "Love is the answer to the darkness." So through Qui-Gon's, Yoda's, and Obiwan's compassion they were able to become "immortal" in the Force.

The following is a question asked to Lucas in the 1989 interview.

What do you feel is the most important lesson you've learned in the last 11 years?

"I've learned a lot of lessons in the last 11 years! (Laughter) Which one is the most important is hard to say. One important lesson I've learned is that the only true route to happiness is through caring about other people. It's true. The truth of it is that it actually works. If all you care about is yourself and about your things and your stuff, you will be unhappy the rest of your life no matter how much you accumulate."


So, all along, Lucas had this personal philosophy of caring about other people, showing compassion. He considers this to be the most important aspect in life. But we wouldn't know for another 16 years that it is the most important thing to a Jedi's soul as well.