In a meta way: the Hermit Gunsmith telling you it's a joy to create when people use his creations to the utmost. Given that Pixel created Cave Story all by himself as a labor of love, it's not a stretch to imagine this is an Author Avatar speaking, about all the enjoyment fans get out of his game.
If you manage to get to the shack in the Waterway with Curly in tow, you'll find a computer inside with a message encouraging you to take a break. After draining her and letting her reboot, the message changes to "You can do it." Fan theories have suggested that Professor Booster wrote these messages and is watching over you and your efforts.
Getting the Iron Bond.
Quote and Curly's past for that matter. Two adventurous robot best friends working together to stop the Doctor, in opposition to the army of Killer Robots.
"My tail wags only for you!"
The good ending, which is only earned by getting the Booster 2.0 from Professor Booster in Mimiga Village, (without talking to him in the Labyrinth and getting the Booster 0.8 first) saving Curly Brace in the Core Room with the tow rope, draining all the water from her body and carrying her to Mimiga Village, and restoring her memories with the Ma Pignon, leading her to giving you the Iron Bond, and heading down the Prefab Building to meet Ballos in his Seal Chamber after defeating the Doctor and the Undead Core. The good ending involves the Island to stop falling and stay afloat, while Quote and Curly Brace are about to be crushed by the Seal Chamber's walls, only for Balrog to rescue them, as per Misery's orders, revealing their Heel-Face Turns when they were freed from the Demon Crown's curse, ultimately being friends with everyone.