Mason was chosen because the original Haley wished for it. He wants Noah to bed and marry him, the only way to make that happen only if Mason is inside his body instead of the original Haley, because Noah only love Mason in the first place.
Okay, that's fair. Though one question, how did Haley know about Mason?
Probably because Haley seems like person who was involved in shaman thingy (I remember there's a scene where Mason in Haley body found the basement and it fulls of magic circle, candles, etc)
my interpretation of the situation was more so that they died at the same time, both having loved or cared abt noah at some point, and like u said Mason is the only person Noah would love. I don't rlly think that Haley and Mason have a connection or any relations to one another which would make sense considering their characters (Mason constantly on missions and Haley being a self absorbed dickwad) they were the complete opposite of each other and the story tries to show that several times. so I don't think Haley actually knew Mason or even of his existence but they both happened to die at the same time and both have a connection to Noah. I'm not sure if the basement stuff was actually important or related but I thought it was to show how obsessed Haley was and how he would do crazy or stupid things js for Noah's attention or wtv
From one comment I thought the body swap really have a plot but it's actually not... Am I just tripping or we really don't know why Mason going inside Haley's body? I mean we knew Haley probably sold his soul or something but we don't know why Mason get chosen