One is the infamous legitimate daughter of the Duke of Guo.
One is a stunningly talented prince
The two got together because of rescuing people. It was only because Yan Jin made rude remarks at that time and Song Xi argued hard that they left a deep impression on each other. After that, a series of stories happened, from not being optimistic about each other to walking hand in hand, and growing in love over time.
Because they have experienced many things together, she trusts him, that's all.
Purpose: Two people for life