Why would one be doubtful and uncertain?

Once the manas accepts a certain principle, then at any time it is absolutely resolute and without hesitation; whereas whatever comes from the deliberation (manasikāra/manana) of the mind consciousness is invariably doubtful, hesitant, and vacillating. It is like riding a bicycle: being able to ride and believing one can ride are two completely different states of mind—the former is without hesitation, while in the latter one cannot be fully certain in one's heart. Some people harbor doubt within but are unaware of their own doubt, thinking they have no questions; that is because their minds are too coarse, their power of introspection (pratyavekṣā-bala) is weak, and their meditative concentration (samādhi) is insufficient. Therefore, when one does not see the true reality, one will deceive oneself.