Why human beings are attracted towards opposite sex?

Since Krishna is the supreme living entity (nityo nityanam cetanas cetananam), we are exactly like Krishna, the difference being that Krishna is Vibhu, unlimited, whereas we are anu, limited. Qualitatively, we are as good as Krishna. Therefore whatever propensities Krishna has, we have also. For example, Krishna has the propensity to love someone of the opposite sex, and therefore we have this same propensity. The beginning of love is present in the eternal love between Radha and Krishna. We are also seeking eternal love, but because we are conditioned by the material laws, our love is interrupted. But if we can transcend this interruption, we can take part in loving affairs similar to those of Krishna and Radharani. Our aim should, therefore, be to go back home, back to Krishna, because since Krishna is eternal, we shall there receive an eternal body.