Recently a friend of mine that lives in Spain told me that he and his wife must renounce their citizenship if they want to be granted Spanish citizenship. This is confirmed by this article and by Wikipedia:
Spain permits dual citizenship under limited circumstances. Spain permits dual citizenship for all Spanish citizens by origin, so long as their declare their will to retain the Spanish nationality within 3 years of acquiring another nationality.
if a U.S. citizen acquires Spanish citizenship, Spain requires the person to renounce U.S. citizenship, but this is usually insufficient for the American to lose U.S. citizenship. In this scenario, Spain will view the person as solely a Spanish citizen, whereas the U.S. will still treat the individual as an American citizen. Practically, the individual will hold U.S. and Spanish dual citizenship.
Foreign nationals who acquire Spanish nationality must renounce their previous nationality, unless they are natural-born citizens of an Iberoamerican country, Andorra, the Philippines or Equatorial Guinea
I am wondering why Spain cares about having another citizenship. If I understood correctly it simply ignores the other citizenship in case of the US citizens.
Question: What is the rationale of not allowing dual citizenship?