Young Golden Blonde Girl Shows Off At The River In The Evening

A theory propounded in The History and Geography of Human Genes (1994), says blond hair became predominant in Northern Europe beginning about 3,000 BC, in the area now known as Lithuania, among the recently arrived Proto-Indo-European settlers (according to the Kurgan hypothesis), and the trait spread quickly through sexual selection into Scandinavia. As above, the theory assumes that men found women with blond hair more attractive.
Social reasons why men may have preferred blonde women is that light hair color is a marker of youth. Since many Northern European children have blond hair, which darkens as they mature, blond hair could arguably be associated with youth and therefore, fertility. Blond as a color has also been associated with light and gold, things that people find attractive and desirable. Lighter color hair is usually more scarce or rare (exotic), arguably increasing its desirability.