Why is taking notes that encourage oneself illegal in chess? In Round 9 in the US Chess Championship 2015 Men, Wesley So (2794) was forfeited by the arbiter in his game against Akobian (2622) since So took notes that were not chess-related but contained "general encouragement and advice to himself".

The game be seen here.

I think that is absurd. I think a chess player can write whatever they want as long as it does not offend anyone.
I can't claim to know for sure what was going through the minds of the authors of the FIDE Laws of Chess, but I think the intent is to ensure that the players use only their unaided intellect to play the game.  Players aren't allowed to make/use written notes during the game.  At all.  About anything (chess-related or not).  If they're allowed to make notes about things other than chess, or the game in progress, you then put an arbiter in the position of having to determine if such notes are chess-related or not.  It would certainly be possible for a player determined to cheat to come up with a system in advance to disguise the nature of what they're writing.

It's worth pointing out that So wasn't forfeited for making the notes - he was forfeited because of rule 11.7 - "Persistent refusal by a player to comply with the Laws of Chess shall be penalised by loss of the game".  He had been warned twice before during the tournament that he wasn't permitted to make written notes, and the second time was warned that a third infraction would result in a forfeit.