I think before a person reached puberty, he/she does not have sexuality consciousness, his/her mind is unconditioned, he/she may admired a certain character he/she witnesses, and tend to like to imitate the person admired, this imitation is very similar to a person undergoes gender change in imitating the behaviour of the real life or imaginary "model" of the sexual identity he/she choosen to conform to.
The question raised is what causes the mind to prefer the particular sexual identity over the other after one's mind has sexuality consciousness, instead of pure admiration of someone [without prior knowledge of sexual preferences] like that of a child. But the main point is that the motive for admiration of both the adult and child consciousness is the same, that is, they both depends on having attached to a particular human model of a distinctive sexual identity.
For an adult person born with a female body whose mind inclined to the self identification as a man, based on the above explanation, this person must necessarily have being strongly attracted to male "model/s", for example, the person has accumulated a habit of admiring a certain male persons, to the exclusion of the female persons. It is very common for an average cis-woman to have a society's conformed habit of admiring other male persons, as the prospect of potential mate. When this habit of admiring a certain male persons, to the exclusion of the female persons accumulates over many years, it become a force within her, in other words, it become a second nature to her to easily like men more than she likes women. If the circumstances is right, and she re-evaluate her self-gender identity, the force of admiring the male "model" will overpower her society's conditioned sexual conformity, if she released her conditioned opposition to this force, it will be her second nature to assume the identity of being male.
The same situation applied to MTF in opposite way.
There are other causes for a person's mind to prefer to play the role of the opposite gender they are born with, these are what the society has conditioned them to believe the advantage vs. disadvantage of the given gender against the other (something which this thread end up discussing), but the resultant decision would be based rationally. I believe one's rationality if relied heavily may well stop the person from undergoes gender re-assignment, within the person there must exist a force much strongly than rationality to enable him / her to willing to overcome much of society's resistance to reach mind's inner peace.