Serena has a crush on Ash. It would make sense that she would do that. Ash, as always, is oblivious to Serena's hints. Serena is traveling to Alola so she can do Contests. Ash is returning to Pallet Town to see his mother. Odds are, Serena and Ash won't see each other again, and with the Sun and Moon series coming out, Ash won't be staying in Kalos, or going to Hoenn with Serena.