That's a good question. The first cause could be the same issues that contribute to forming a homosexual orientation, such as neglectful same-sex parenting, peer rejection and sexual abuse. Second, one may participate in bisexuality because they were introduced to that kind of behavior. Sometimes when parents don't provide any healthy guidelines or morals to follow, when there are opportunities for bisexuality, they may experiment with sex with both genders. Third, an individual may be drawn to bisexuality because they are narcissistic; they want both sexes to find them appealing.