The world was crashing down around them, and he was running off to meet it head-on. She wanted to shout to him, tell him to stop, but to be honest, it was pretty much up to him and whatever duelists that were left standing to save the world. He knew perfectly well that he could die, and he was willing to go anyway.
Which was exactly why she wanted to hold him back. Jounouchi was far too young to throw his life away for some vague ridiculous Egyptian enemy with a grudge on Yuugi. But he was running towards the monsters that were swarming and ready to let them kill him if he could do anything to help.
Mai found herself being carried forward. She was running after him. If he wanted to throw away his life, fine, that was his descision. If she wanted to follow him because she was in love with him, and throw away hers, too, that was her descision.