Marner is clearly not happy with all of the trade rumors that he has been involved in lately, which has been a major distraction for him this summer. Another possibility is that Marner knows some information that others don't, which is that he is well aware he will be traded at some point next season, but doesn't want to risk ruining any potential trades, so is choosing to remain quiet.
Marner's value is extremely high, as he just came off a fabulous season where he put up an astounding 85 points in 69 games, leading many to believe he would have eclipsed the 100 point mark had he played in his team's additional 13 games. GM Brad Treliving can get quite the haul for Marner should he decide to pull the trigger on a trade.
Marner has always been a scapegoat in Toronto, taking heat from fans and reporters for many years, and that will only intensify as the weeks and months go on, as trade talks will surely continue to heat up.