That gnawed at me. There was both truth and problem in that... And I thought about it.
True, I cannot be held responsible for a player's fun. Each person brings to the table something, and what one may love, another may not. Real-world issues sometimes bleed into the game. There is no way for me to know. I can help, but in the end each player is responsible for his own enjoyment of the game.
So after that, I thought of a number of scenarios when I did not enjoy myself. And I came up with the caveat of the original assessment.
While I cannot be held responsible for your fun/enjoyment, I can must be held responsible for preventing your from enjoying the game.
What I mean here is that through action, inaction, nagging, or out-of-game, I can make it so the enjoyment of others is significantly lessened. And that, I can be responsible. I joked that which I cannot be responsible for you breathing, by putting my fingers around your throat, I can make it really hard for you. Yes that's was a little assassin humor! Unlike many players, I like to create characters with a blaring flaw or a built-in problem: whether its lack of melee abilities or a very low attribute. Something. Also my character should be good at what I want to do: melee, spell casting, healing, or ranged attacks. That's the character's role and goal. From that base he can and will florish in the game.
Back in college, I used to do improv-theater, and one of the common penalties was "character refusal". Character refusal was a way of blocking others rather than roll with the punches. It usually had the unfortunate side-effect of stopping or blocking creativity. It wasn't simply saying "No". As you entered play, someone told you "Hey John the Fisherman!" And you replied "I'm not John and I'm not a fisherman" was surefire penalty. However, replying "I'm Bill, John's twin and I'm looking for John" would not. In an RPG, players have their characters pre-established, but players - and I've guilty of this myself - sometimes block each other's actions. Shall I point out the two trapspotting rogue parties? Who disables the traps? "I have +10, you're +9 you help me..."
I'm sure of it. While I cannot make it so someone enjoys the game, I can make it so someone does NOT enjoy the game. Not an easy thing.