Why is the Buddha so confident?

Ven. Nyanatiloka, Buddhist Dictionary (palikanon.com); Dhr. Seven (ed.), Wisdom Quarterly

The first human depictions of the Buddha were Greek from Gandhara, Central Asia.

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The "self-confidence" (vesārajja) of a Buddha is fourfold. He is confident:

1. to have attained to a perfect enlightenment of which it cannot be said that it omits anything essential to it;
2. to have destroyed all defiling taints (āsava), leaving none that can be said to be undestroyed by him; 
3. that what were declared by him as obstacles to liberation are undeniably such;
4. that his Teaching (Dharma) fulfils its purpose of actually leading to final liberation (nirvana) from all suffering.

(See A.IV.8; VII.58; M.12).