[Clarste]

Originally Posted by Oroboro

Spoiler for Answer:

Try thinking of it in less terms of "Awesome gamer who beats a difficult game" and more "Soldier coming home from a war." He certainly had his moments of peace for awhile, but he's also been fighting for 2 years, and has seen a lot of people die. Furthermore, I feel like it'd be difficult for anyone from the outside to understand, since from their perspective, it was just a game, and all they saw was the coma, not the life and death struggles that went on each day.

Not to mention with Asuna still in a coma, you could say Kirito's good memories of Aincrad are still locked away as well.

Spoiler for elaboration on answer:

Plus, you know, there's the part where he killed a man. In self-defense, sure, but it's not exactly the kind of thing you would feel comfortable talking about.

"So how did you get together with Asuna?"
"Oh, she had this stalker guy who poisoned me, killed an innocent bystander, and then almost killed me before Asuna came and rescued me so I had to kill him with my bare hands. Then we slept together and I proposed marriage."
"..."

Not to mention all the parts where he lets people die. There's a lot of guilt in his story. Even taking only the good bits, it's hard to mention them without feeling like you're intentionally omitting the other parts.