got this from wiki
In the NFL, the attempt for extra point(s) is required after a touchdown scored during the regulation (i.e., not overtime), because points are used for some tiebreakers in the standings. Rarely, this can result in such an attempt having to be made at the end of the game when it cannot change the outcome of the game. If the game is in sudden death overtime, the extra-point attempt is omitted if the winning score is a touchdown. In American high school and college football, it's likewise omitted following a touchdown on the game's final play if six points were enough to win or if the scoring team was already ahead.