I don’t think they dictate, keefe ultimately makes the decision but I think everyone knows they want to play together and prefer to play together.
In 2017-2018 - Matthews played on line with Hyman, Brown, Nylander. All three of his linemates in their second NHL seasons.
Meanwhile Marner was playing on lines with veterans Marleau, Kadri, JVR and Bozak. All with exception of Marleau in the primes of their careers. Marleau was coming off a 27 goal 46 point season.
In 2018-2019 - Matthews primary linemates were Kapanen, Johnsson, Marleau and Nylander. The line with Nylander being the least of the main three. So the oldest guy on the roster and two rookies.
In 2018-2019 - Marners primary linemates were Hyman and Tavares by a wide margin. The next biggest being Johnsson Tavares. So he's playing with a two time Hart trophy nominee in his prime and Hyman who was entering his third full season.
Can't really blame Matthews for making requests, he was clearly being given the short end of the stick in terms of linemates by Babcock. Heck both Matthews rookie linemates in 2018 (Johnsson and Kapanen) had their best years with him and then struggled to stay in the league afterwards.