Romantic Chemistry

What really is romantic chemistry? Attraction? If it is an illusion, why is it so important to have it with a partner? What if you recognize a good person with whom you have no romantic chemistry, is it worthwhile, in your opinion, to make a relationship happen with this person? Is it better to choose someone who is a wonderful person, even if there is no attraction, because chemistry dies eventually anyway? A best friend rather than a lover? Thanks for answering my question.