This (Dissimulation and falsity of speech) is an exceedingly injurious habit of the lower nature. Those who are not straightforward cannot profit by the Mother’s help, for they themselves turn it away. Unless they change, they cannot hope for the descent of the supramental Light and Truth into the lower vital and physical nature; they remain stuck in their own self-created mud and cannot progress. Often it is not mere exaggeration or a false use of the imagination embroidering on the actual truth that is marked in the sadhaka, but also a positive denial and distortion as well as a falsifying concealment of facts. This he does sometimes to cover up his disobedience or wrong or doubtful course of action, sometimes to keep up his position, at others to get his own way or indulge his preferred habits and desires. Very often, when one has this kind of vital habit, he clouds his own consciousness and does not altogether realise the falsity of what he is saying or doing; but in much that he says and does, it is quite impossible to extend to him even this inadequate excuse.
Ref: Letters on Yoga – IV