In “American History”, why is Elena so happy on the day of President Kennedy’s death?

In spite of the sadness that engulfs El Building and the surrounding environs on the day of President Kennedy’s death, Elena is happy because finally, Eugene has invited her over to his house. He wants the two of them to use the time to study for an American History test. The two also plan to walk together to the public library, later on. It appears that Elena is infatuated with Eugene. The story states that “there was only one source of beauty and light” for Elena that school year. That was “seeing Eugene” every opportunity she could. Elena says that “she had liked Eugene right away” because he spent most of his time reading at their kitchen table. She spends a lot of time watching his house from her bedroom window. In fact, she had liked him so much that she sought him out at school to introduce herself. Afterwards, the two of them walked home together most days.
However, Elena’s happiness on this particular day is short lived. When she gets to Eugene’s house, his mother sends her away, saying that Eugene certainly does not need her help in studying. She asks her to go back to her own house.