What is behind the irrationality that seizes otherwise reasonable human beings, forcing them to set aside other goals and strivings and to focus on a single other individual , who may be of little interest prior to limerance and also of little interest afterward?

I recall reading early in the book about and identifying with  Love and Limerance having thoughts, even fantasies, of tragic events bringing two people together. On the ride home today, I was thinking about the americanized version of High Fidelity, and how the death of Laura’s father brought her back to Rob. I’d be really interested in why the choice was made to write the script that way, doubly so because it is so different from the original book.