In a dim light, details become more vague. Flaws can be easily ignored, and virtues imagined. Not only does this perhaps help to see the partner as a bit more splendid than you might in a bright light, but more importantly it helps make you a bit less self conscious about your own flaws.
Second, a dim light suggests privacy, suggests that nobody is watching, suggests that the world is just you and your partner alone. Perhaps an artifact of a distant time when humans always lived in groups, and might want to wait until the others were asleep, or to enjoy each other before the others awoke.
Third, and this one is the most scientific of all, there is an interesting physiological effect. In dim light, the pupils dilate, which is to say the black dot in the center of your eye becomes very large, so as to capture more of the available light. In bright light, this dot contracts. Well, as it turns out, in unchanging moderate light conditions, people who are very interested in what they are looking at will also have enlarged dots, while people who are not will have relatively small ones. Part of our very complex innate system which allows us to read each other’s faces or moods includes this observation, so you subconsciously register the large dot as interest. People who are interested in you are interesting To you.
Fourth, and least important, we’ve been socialized to see dim light as a signal that romance is intended. A self fulfilling prophecy, if you will.
I think only the third really qualifies as scientific, but really all are psychological.
