I don't know anything at all about my opponent. I don't know if he plays better in open positions or closed positions. I don't know if he prefers e pawn, d pawn, c pawn opening, or something more offbeat. I don't know if he prefers classical or hypermodern. Now if I knew that I was playing a stronger opponent who didn't have very good mastery of hypermodern, then I would play hypermodern. But most of the time we don't know...and this is partly where the element of luck comes in.
You have a point here...But whatever your opponent does you can always respond and plan ahead.You remind me of sports.For example a basketball coach has made his plans before a game,but does not know what is in the opponent's mind.However,he can react during the game against the other coach.Of course,players' performance may vary from game to game,but chess pieces are always the same!Athletes with more skills sometimes lose from others with poorer perfomance,which can also happen in chess.I always keep in my mind the fact that nobody can remain at his/her top performance every day,which stands for athletes,workers,doctors,writers,everybody.
My friends,for me,luck in a chess game is:
a)the opponent's blunders
b)the opponent's accidental poor performance or 'bad day',as we say in Greece
c)the opponent's unwillingness to think or concentrate at the moment
d)your personal condition during the game
But we can't say that a 2200 player has been luckier than a 1200 in a lifetime!!!
Luck CAN play a part in this...
Lucky for you is when your opponet makes blunders