>>57786505Satoshi was extremely pumped up to train for his battle with Valerie, especially after seeing Shota get bodied by her earlier in the episode, so he quickly downed his dinner, ordered his mons to quickly down their dinner, and immediately declared that it was time for late-night training! Shota then offers to train with him, and Satoshi happily takes him up on the offer, and the two leave the table. Serena, who's been showing an especial endearment toward Satoshi's impatient quirks all episode, remarks that he can't even eat patiently when his battle autism consumes him... but because she's in love, she feels the urge to copy him (hence her also eating her dinner fast) and join him and Shota to root them (well, root for Satoshi at least) on for their battle. Not shown in the screencaps anon posted above, but Bonnie and then Clemont soon follow suit and also speedily eat their rice to join the others.
tl;dr: Because she's in love.
