There are a number of theories about why transgender people exist although there is not yet scientific consensus. When you look across cultures, you will find that people have had a wide range of beliefs about gender. Some cultures look at people and see six genders, while others see two.
Some cultures have created specific ways for people to live in roles that are different from that assigned to them at birth. In addition, different cultures also vary in their definitions of masculine and feminine. In some places, men are expected to be gentle and nurturing, while in others, they are expected to be strong and stoic.
Whether we view someone as transgender depends on the cultural lenses we are looking through as well as how people identify themselves. Biologists tell us that sex is a complicated matter, much more complex than what we may have been taught in school.
A person has XX chromosomes is generally considered female, while a person with XY chromosomes is generally considered male. However, there are also people who have XXY, XYY, and other variations of chromosomes; these genetic differences may or may not be visibly apparent or known to the person.
Some people are born with XY chromosomes, but are unable to respond to testosterone and therefore develop bodies with a vagina and breasts, rather than a penis and testes. A variation in gender may just be part of the natural order and there are more varieties than we generally realize. People with biological differences in gender may be considered inter-sex; they may or may not identify as transgender