Why are some people bisexual?

No one knows ... not withstanding the 8 thousand excuses learned men give.

It probably has something to do with hormones, but hormones are mostly genetic;

and it probably has something to do with their environment, their upbringing.

Answer 2:

Just like some people are only sexually attracted to people of the opposite sex and others only to people of the same sex, some people are sexually attracted to both genders.

Note that this does NOT mean that bisexuals need a male AND a female partner. That's not how it works at all. It just means that the one they fall in love with can be either a man or a woman.

I must also say that the person who gave the first answer is wrong on the last part. No one's sexual orientation can be influenced from the outside. It is true that a bisexual who is raised in an environment where it is seen as bad, may deny it or simply not realise it is possible to truly feel that way - and that someone who is taught that it's okay to be bi, may come out of the closet sooner and more easily, because they recognise what they feel as something acceptable. However, upbringing does not influence or change your sexual orientation in any way. It may influence who you actually have a relationship with, but it doesn't change your feelings inside.

Remember, your sexual orientation is not about behaviour - it's about feelings. You're just born with it.