"A man convinced against his will is of the same opinion still."
--Dale Carnegie
I won't describe this in detail, the post will lose its purpose.
When trying to sell anything, don't convince anyone they need it for they probably don't know "why" they need it.
I know what you are thinking. No, don't "tell" them why they need it.
Help them see it. Ask them questions. Let them discover the "why" themselves. You won't even need to sell it. If "why" isn't strong enough, the sale wouldn't have happened anyway (maybe once in a while if you are lucky, not repeatable).
Would love to hear different perspectives on this quote from the community!