Why does Jia Baoyu love Sister Aileen instead of Sister Bao?

Your question is equivalent to: After Jia Baoyu falls in love with Sister Bao, you will ask, Why does Jia Baoyu love Sister Bao but not Sister Lin?

Then I'll tell you, Baoyu is affectionate, Daiyu is affectionate, Baoyu is delicate, Daiyu is sensitive, Baoyu is simple, Daiyu is persistent, Baoyu is affectionate, Daiyu is affectionate ... They have these feelings that complement each other, which an intelligent, eloquent and sophisticated treasure sister does not have.

A word of advice: don't dwell on why he fell in love with her but not with another one.

Love depends on sensibility, not reason.