I've always personally preferred real blondes because my wife is one; so much like my leanings towards older women, she has influenced me. I guess I find them most intriguing because when I see one, unless it's a noticeable dye job (the roots are showing or the eyebrows don't match), there is always the question and arousing thought of
how blonde they actually are. This will always be one of my most memorable surprises when I first had sex with my wife in that not only did the carpet match the drapes, but was even much, much lighter than them. I have found this to also be the case with maybe half the blonde women I knew when we were in the Lifestyle and the anticipation of solving the mystery was likely a good part of the appeal, while with brunettes and dark-haired women, what you saw was what you got (assuming they didn't completely shave).
I think several of the other reasons a good deal of men prefer blondes has already been mentioned, like association with youth and innocence to the point they even named a movie based upon the average guy's preferences

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Though as mentioned above, natural blonde hair does tend to darken with age (and with women, pregnancy) and one of the reasons it is more representative of younger women and men alike. Not only was I a complete towhead up until my teen years and am now completely salt & pepper, but even my wife if she stopped dying completely would be more a brownish brunette, though thankful that still hasn't taken hold everywhere

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