Messi is known as one of the best player in football history, the retirement from international football came after when he missed a decisive penalty and Argentina lost final against Chile on penalties in Copa America 2016. Here are some possible reasons why he did so:
After the match in a press conference messi told reporters "It's been four finals, I tried,It was the thing I wanted the most, but I couldn't get it, so I think it's over".
It is evident by his statement that he was fed up by the defeat in four finals as a captain including FIFA World cup 2014 final against Germany.
"I think this is best for everyone. First of all for me, then for everyone".
I guess He was depressed by the four defeat in major final and lost hope that he could ever win a major trophy for his country
- "I think there's a lot of people who want this, who obviously are not satisfied, as we are not satisfied reaching a final and not winning it."
messi is continuously compared with Diego maradona, but he was not able to win the world cup or any other major title for his country, so there was also some criticism by Argentinian crowd , that he can never be the Maradona for Argentina despite winning five ballon d'or (Fifa Best player of the year). This debate heavily came in to existence after defeat in Fifa world cup 2014 against Germany.
These are the 3 major possible reasons why he took retirement ,although he will continue to play for his club FC Barcelona.