Unfortunately, there are very few theories that show any significant evidence as to why some people are homosexual. There are, however, many biological and physiological studies that show some possible links and may contribute to an individual's sexual orientation.
First of all, research studies have shown that there is a strong genetic link in homosexuality among males. While evidence also indicates that there is a genetic link in homosexual females, evidence has been stronger in male studies. This would indicate that genes play a possible role in the development of sexual orientation. Other studies suggest that hormones may play a part in determining an individual's sexual orientation, especially during the fetal period of development. It is possible that homosexual females are exposed to higher levels of testosterone during the fetal stage while homosexual males are exposed to higher levels of estrogen.
Most interestingly, a scientific study of the brains of homosexual and heterosexual men showed that the anterior hypothalamus (the part of the brain that influences sexual behavior) in gay men was much smaller than in straight men. Later studies showed that this region of the brain was larger in homosexual women than it was in heterosexual women. While this does not provide a sufficient explanation for homosexuality, it may contribute to an individual's sexual orientation.
Most researchers agree that sexual orientation is the result of both nature and nurture. An individual may be born with certain tendencies and predispositions which can be fostered by their environment and result in a particular sexual orientation. This has not been proven, but it is believed to be a fairly accurate hypothesis.