It comes down to trust and expectation. Everyone can say they are passionate, interested, want to do the hard work to make the mentor's investment in them event

It comes down to trust and expectation. Everyone can say they are passionate, interested, want to do the hard work to make the mentor's investment in them eventually worth it. Those are very weak signals. On the other hand, actually having demonstrably put in a very significant amount of effort already is a strong proof you are being serious.