Even though [the Buddha-Nature] is our basic nature, we do not realise it; we do not manifest the Buddha's qualities openly, because we have not purified the temporary stains of ignorance that obscure our realisation. This is what causes us suffering. However, since these stains are not inherently a part of mind's nature, they are suitable to be removed, and practising on the path accomplishes precisely that. When all the stains have been purified, one attains the "Dharmakaya free of fleeting stains."

-- Khenpo Tsultrim Rinpoche