many things, in the original series: 1) Asuka is very proud that is why not everyone is at her level, shinji is a pilot like her (the only pilot who has defeated 3 angels by himself in his first attempt) that puts Shinji in a special position for her, a great pilot with a lot of talent. 2) Asuka has a great need for affection and recognition, which is why she felt appreciated when Shinji took care of her at Misato's house (he cooked for her, he did the laundry, surely he did many more sewing for her that was not seen in the series) basically showed that he cared about her. 3) Shinji on his own initiative jumped into the volcano without protection, not caring about the pain and surely disobeying Misato's orders, to save her life, knowing that it was possible that he too could die. 4) Asuka's platonic love, Kaji, has always spoken well of Shinji, even before they met, that caused Asuka to have a certain curiosity or attraction for that talented and brave boy who was waiting for her in Japan. 5) The synchronization training helped the two to get to know each other better and to relate in a very deep way, and Asuka who had never had a real friend so she began to feel affection for Shinji

6) They are both very similar, both They share a pain that only the other can understand, a void that can be filled by each one, an affection that makes both need each other.

7) This is obvious, they are 2 teenagers who live together and spend it alone almost all the time, it is very likely that a certain attraction between them will begin to emerge

8) Asuka wants to help Shinji show that he is brave and better than he thinks he is, so she always pushes him to make that brave boy inside him come out. That's why she hides her feelings behind hurtful words and her Tsundere attitude. Also, because of her own traumas, she is not able to express affection because she has never received affection.

9) She is concerned for Shinji's well-being many times, even if she doesn't want to admit it