Why go so far as Shechem and Amnon? “Vaye-ehav Yaakov es Rachel.”
Real love develops with time after marriage, but for most of us the beginning of that emotion starts initially with some feeling of attraction that enables the couple to want to get to know each other better for purposes of getting married in the first place. Certainly Hashem built into the concept of continuation of all life human or not, some chemistry between the two individuals involved.