Why do Tulku/reincarnate realized masters have do relearn Dharma, when they take a new incarnation.
There are basically two explanations for this. For tulkus at a lower level (so below 8th bhumi) they don't retain knowledge from lifetime to lifetime. They do build up merit and wisdom and have deeply ingrained positive habits but upon death all their actual conceptual knowledge (or most of it) is forgotten although some remember parts of it. Also lower level tulkus can also be tulkus who aren't even at the 1st bhumi yet and they are just getting by on prayer and aspiration and have even fewer positive habits and even more kleshas to deal with.
Tulkus on the 8th bhumi and above are said to basically be going through the motions of relearning.
You hear stories, and hear from some tulku's that they are just some ordinary guy. There was some recent news about the current kalu rinpoche going to a rough period(understandable though) and had a brief stint with drugs and alcohol. Shouldn't they by the very nature of being tulu's always abide in Nirvana, and be beyond suffering, since realization is supposed to be permanent, and that is afterall why we strive for it no?
Actual realization is permanent. But most tulkus aren't realized. The lowest level tulkus purified lots of karma in previous lifetimes and accumulated merit and some wisdom but took rebirth on the basis of prayer and aspiration and the force of the Bodhisattva Vow. Tulkus on the lower buhmi's do have realization but can still screw up. Also realization does not mean that they can conceptually explain the teachings. As kids they know the teachings intuitively but can't explain them conceptually. Once they rehear the teachings then things begin to click and they can advance quickly (this can also be true of tulkus not on the bhumis).
So some good sources are Berzin, "Enthronement, The Recognition of the Reincarnate Masters of Tibet and the Himalayas", Ngawang Zangpo (Hugh Leslie Thompson) and Tulku Thondrup's "Incarnation".
When tulku's or reincarnate realized masters speak of inner difficulties, there is course a sense of humbleness, but It also gives me feelings of discouragement.
As long as we have a body we will have obscurations in some form. The higher tulkus mostly have difficulties from circumstances. Mostly this means that spiritual forces are trying to keep them from accomplishing what they need to.
I've been questioning of this too. The story of 17th Karmapa who loved to eat meat when he was a child. And later, by the influence of other Master, HH began to adopt vegetarianism.
Tibetans have not commonly been vegetarians so this habit would not come from previous lifetimes for tulkus.
Also Why HH did not remember other thing related to the Buddhist practice (has to be taught by his teacher) BUT can remember the good belong to 16th Karmapa e.g. Mala, Bell, etc?
I covered this above. Now a strong practitioner could probably identify their mala, vajra and bell from their previous lifetime because they would have such a strong connection with them.
But the Tulku reincarnation, to me, is questionable. Is the purpose of reincarnation is just a reincarnation or there is other purpose? Any thoughts?
The only purpose of a tulku's reincarnation is to benefit beings and the Dharma.
_________________Kirt's Tibetan Translation Notes
"Only you can make your mind beautiful."
HH Chetsang Rinpoche