"It's not going to work!" "Oh, yeah?" "And why's that, Granger? This is an age line. Dumbledore drew it himself." "So someone like Dumbledore couldn't possible be fooled by something as daft and dimwitted as an aging potion"
I think it's because Dobby comes to Harry's aid when he needs it. I loved how he called Ron Harry's " Wheezy", the one thing he would "sorely miss". He would do anything for Harry, and now Ron. "Dobby cannot let Harry Potter lose his Wheezy!