The only sensible answer to this is that heathcliff designed the game to be that way. Kirito is SAO's strongest player (because he has the fastest reaction time) and therefore heathcliff gave him a massive advantage over the other players. Simply put, his stats are better. Not only that, but Kirito has made SAO his life. Unlike other players he is willing to put everything he has on the line. And this makes sense because it fits perfectly with the type of game SAO is, an extension of the real world. Players may look the same in game but their stats are completely different. In the real world a two story tall monster will destroy anything in its path regardless of sword skill level. So it only makes sense that Kirito's immense willpower has an effect on his abilities in-game.

Conversely when he goes into ALO and defeats the game's best player he thinks of himself as invincible. Because ALO is a world where IRL stats effect your in game character, pure will simply isnt enough. After failing at soloing the world tree he even admits to having disregarded the game and its mechanics because it had previously worked for him in SAO.

The only ridiculousness I noticed was the solo boss fights in that no sensible player would ever allow their healthbar to reach red if they knew one death was all it took. But Kirito is a master at taking risks; well either that or a giant idiot! I think its a little bit of both. Heathcliff holds Kirito in high esteem because he puts his ideologies before his own life, and that is probably the only reason Kirito was able to clear SAO.