Messi's only 29, it's just an emotional reaction after losing the third major final with the national team in the last three years. He won't miss one (possibly) last opportunity to win the World Cup.
Mascherano on the other hand will be 34 in 2018, he may have had enough.
I hope that Messi reconsiders but he may not.
He's very independently-minded and naturally aloof, apparently something of an enigma even to friends and family.
He's had a difficult relationship with the Argie FA and the Argie people for several years.
This article about Messi from 2011 is interesting:
http://espn.go.com/espn/eticket/story?page=Lionel-...
'...friends and family admitting that Messi is unknowable even to the people closest to him. "When he's not doing so well," close friend Cintia Arellano told the writer, "Leo is a little bit solitary. He retreats. He withdraws into himself. He was like that even with me sometimes. It was like drawing blood from a stone trying to find out what was going on inside."
Having left Argentina aged only 13, it seems he doesn't identify with it, nor it with him.
Messi never sings the national anthem before games and has said it means nothing to him - which only angers many Argentinians.
He is not especially a hero in his home town Rosario - even in a bar owned by his family, there is nothing identified with him and they don't necessarily screen his Barcelona games. The locals have always been indifferent to him.