Why did Shlomo Hamelech marry many wives? Being that women naturally want to be the "only one"--from the fact that co-wives would fight each other for the love of their husband as the Mishna (Avot 2:7) says, "מרבה נשים מרבה כשפים (more wives=more witchcraft)" (thanks Fred for the source) and as the Talmud relates (co-wives are called tzarot, which relates to pain), as Song of Songs 6.9 hints to, as evidenced by polygamy being illegal in most of the world under grounds of women's rights violations, as intuition and the spirit of the time shows, and as claimed by many women today through various forms of media--what reasoning would cause the "wisest of all men" to marry/love more than one wife?
(This question can apply to other righteous people like King David, but it is strongest in this case, because King Solomon may have known things that people in his and other generations did not. [Though the simple explanation is that every Jew knew about the Mishna in Avot and the negative outcome of such a marriage, so the question can be why anyone who would risk such a relationship]. Also, I am assuming that there were enough potential husbands available in Israel, because the Jews were wealthy during his era as stated in Book of Kings 1).