Serena became infatuated with Ash when she became lost in a forest while attending the Pokemon Summer Camp organized by Professor Oak as a child. Ash helped her and escorted Serena out of the forest. He gave her a handkerchief to which Serena kept to this day.
When Ash came to Kalos, Serena began her journey only because she wants to meet up with Ash. After meeting up with Ash, she quickly joins them, primarily because she is in love with Ash.
Theories that she is a possible yandere emerged when her obsession over Ash grows evident as the season progresses. She showed jealousy when Miette made advances towards Ash, and was furious when Ash accepted to dance with Miette in Party Dancecapades!.(Ash saw her anger, but not Miette)
In XY115, she went as far as to battle Jimmy with Pikachu while Ash was sick, even taking his clothes and dressing up as him, because Jimmy wanted to fight Ash in a Pokemon battle with his Pikachu. Surprisingly, she managed to copy Ash's battling style.
If Serena ever met Misty, she would probably kill her when she figures out that Misty likes Ash. However Misty is more tsundere, making Serena and Misty possibly similar to Ayano Aishi and Osana Najimi from Yandere Simulator, respectively.
Though, other points say that Serena really just has a big crush on Ash.