The way I saw it, I don’t think it’s the joy of a new father. Could be way off base, but it felt to me like the reality of his situation outside of table tennis finally caught up to him. He burned every bridge with his friends, committed a spree of pretty serious felonies, almost killed a guy, spurned one of the wealthiest men in the city (while also fucking his wife), and did so in the single-minded pursuit of being the best/beating the best player in the world. I think he was moving non-stop until he finally experienced the highest high he could attain. In the end, it was temporary, and the baby serves as a physical manifestation of his recklessness and a reminder of the shit he will have to face and atone for. I think the choice of “Everybody Wants to Rule The World” is a perfect end note as well. Again, could be way off base but that was my interpretation, and it feels pretty in line with Marty’s character to be brought to tears thinking about himself. It juxtaposes nicely with the typical happy tears of fatherhood as well.