When Jedi get older and wiser their perspectives change entirly. And it is brought up in a point in The New Jedi Order book Edge of Victory: Conquest. Kyp Durron says that when Jedi Masters get older they just battle between the idea of Light and Dark and the myth of the Force and never get anything done. I dont agree with that completly but it seems true. In the PT Yoda is already 800 years old. In almost every scene he is in he is making a reference to the Light side or the Dark side. Obiwan in the PT is much more rougelike and isn't scared to go hands on. But in the OT at his old age you can tell he is more wise and more attuned to the force than ever before. Hence letting himself die to become one with the Force. All of that just to fulfill a beleive he had about bringing balance between the darkside and lightside. Later on in EU Luke has established his Jedi Order and prevents hands on action many times because of the balance point between Lightside and Darkside.