[Update: and now there's a fifth discussion thread, which you should probably use in preference to this one. Later update: and a sixth -- in the discussion section, which is where these threads are living for now on. Also: tag for HP threads in the main section, and tag for HP threads in the discussion section.]
The third discussion thread is above 500 comments now, just like the others, so it's time for a new one. Predecessors: one, two, three. For anyone who's been on Mars and doesn't know what this is about: it's Eliezer's remarkable Harry Potter fanfic.
Spoiler warning and helpful suggestion (copied from those in the earlier threads):
Spoiler Warning: this thread contains unrot13'd spoilers for Harry Potter and the Methods of Rationality up to the current chapter and for the original Harry Potter series. Please continue to use rot13 for spoilers to other works of fiction, or if you have insider knowledge of future chapters of Harry Potter and the Methods of Rationality.
A suggestion: mention at the top of your comment which chapter you're commenting on, or what chapter you're up to, so that people can understand the context of your comment even after more chapters have been posted. This can also help people avoid reading spoilers for a new chapter before they realize that there is a new chapter.
The may be ok advice and perhaps worth a shot. It may even work - in a fantasy story. But real people tend to have better (or, rather, stronger) social and psychological boundaries - it is actually hard to exact fundamental personal change from people just by approaching them in a friendly manner. And giving unsolicited brutally personal advice to people actually isn't a reliable way to gain friends.
Not Quirrel's. Not like that. Quirrel's advice pertained to an entirely different sort of influence than what you and Flitwick suggest. With Quirrel's Slytherin-typical strategy you influence by controlling the political, reputational payoffs. Direct heart-to-hearts are completely opposed to the spirit of it.
I also suggest that "self-centredness" is not the relevant flaw of Harry's here. This is actually a situation where more self-centredness would have prevented the err (such as it was). Harry has blurry boundaries on just what he is optimising for. Is he optimising for his self, is he optimising for blades of sentient grass or is he optimising for what Hermione might call "her own business"? People don't tend to like it when you act to control things that they don't perceive to be 'yours' - even if, as in this case, it is a benefit to all concerned. A self-centred Harry would have made entirely different mistakes to boundariless-Harry.
It is also -- outside fiction -- not a reliable way to get people to follow that advice.
Neither is offering friendly advice. Or, for that matter, advice of any sort, however delivered.