Are these meanings that arise in the soul upon success or failure in da'wah, and in the event of a large or small audience, detrimental to sincerity, or are they part of human nature?

What the speaker finds within himself after delivering the sermon is either a joyful anticipation of Allah's blessing, delight in His grace, and hope for His reward—and this is among the signs of faith. Or it could be self-admiration and an inclination towards people's praise, which is a destructive affliction that falls under blameworthy ostentation (riya'). This self-admiration can be dispelled by remembering that the blessing is solely from Allah's grace.