Ruan Zhiwei picked Shen Qingyan because that person was "clean".
The second young master of the down-and-out Hou Mansion was talented, warm-tempered, and most importantly - he had no power to manipulate her. By marrying, she can truly "get out of the house" instead of changing to a more exquisite prison.
She began to "appear just right" at small banquets. She didn't say much, and only her eyebrows and sense of proportion were remembered.
What Pei Zhiheng saw was another thing: she was paving the way for herself, and the end of the road was not at the Hou Mansion.
He doesn't like this lack of control. It's just that he didn't admit it at that time, and just attributed it to: "Trouble."
Her plan: trade beauty and dignity for a way out.
His misjudgment: She is something "can be picked up casually", so her abacus should also be his.
Intention: He thought she was just a "toy who could serve others". She almost got married before he was willing to kneel down in the dust.