Bowser just flat out kills Luigi at the start of the game to be re-established as a threat.
Nintendo consider this canon and Luigi never appears in any Mario game again aside from one cameo apperance in Waluigi's mansion as a ghost in which he is captured without much fanfare near the start of the game.
How would you react?
What would you do?
^ Hey now that's completely unfair!
http://i.imgur.com/yPw05Ob.png