Notwithstanding your (questionable) premisessecond premise ... :)
I'll say that not all Tibetans are such sweet souls and you'd be hard pressed to find many that are anywhere near as incomparable as HHDL... but the answer is simple: practice of the Dharma.
His Holiness is a Bodhisattva who has been wholeheartedly practicing according to the Bodhisattvacaryāvatāra for many, many, many years if not many, many, many lifetimes. What you are seeing is the result of such continued and amazing practice. Consider his Holiness's favorite quote which he repeats constantly during his teachings:
With a wish to free all beings I shall always go for refuge to the Buddha, Dharma and Sangha until I reach full enlightenment.
Enthused by wisdom and compassion, today in the Buddha’s presence I generate the Mind for Full Awakening for the benefit of all sentient beings.
As long as space endures, as long as sentient being remain, until then, may I too remain and dispel the miseries of the world.
If one faithfully practices and generates this Mind of Enlightenment the results will follow. You are just observing the results.