The thirty-one year old fought a nine year professional campaign, going 19-3 with twelve kayos and world championships in two divisions (IBF straw with two successful defenses and WBA Super / Ring Magazine light-fly with four) - trying for a third in his final outing. Avenged one of his three losses. Not too shabby.
I would've thought if anything one more go with Olascuaga (fan scores appear to be all over the place on that one) before calling it a day but...end of the day, fighters knows themselves and when it's time.
I'm sure Kosei Tanaka's messed up eye, Anaguchi dying, the suicide of Sakama, Shigeoka having brain surgery and the brutal KO loss of Sasaki, all factored into his decision. It's been a rough few years for Japanese boxing. It's a real pity, because win or lose, his matches were always usually a thrill to watch.
A poetic soul might be inclined to say it's like the boxing world has been demanding sacrifice from the Land of the Rising Sun as the price of having two massive talents be top 15 p4p simultaneously.
Quadrilogy with Paras would have been cool too. He may reconsider as he left the sport on his own volition unlike Tanaka.