Purity. The purer the mind of the speaker, the more purely unconditional love will be expressed through them. And for this purity to have happened, they must have had the purest intentions and honored them devotedly until it happened. So, I feel the inspiring element in a motivational speech is the speaker’s being expressed through a purified mind.
After this, devotion. Devotion can take a person to purity even though they may not be at the purest state of mind while giving the speech.
After this, sincerity. Even though while giving the speech the speaker may be ignorant, their sincerity will reduce their ignorance even while giving the speech. Eventually this sincerity will lead them to devotion.
I think purity, devotion and sincerity are inseparably linked. They are just different stages of realization in our minds of the core of our own being. And the real source of inspiration always is the core of our own being, it is our own being. From a human perspective, it just shines through in different places of existence and inspires our person, and it is always available in the core of our person as well.