Users often ask similar questions and are welcome, graciously, with validations - often in the form of "you feel this way because you are trans" or "these feelings you've described are trans feelings". These are important validations and my hope for you is that you can feel supported and loved by an online community, if not an interpersonal one too. This validating is absolutely necessary, especially given then immense rates of self-harm, suicide, interpersonal violence, and institutional oppression.

However, your question "Why do I wish to be a girl?" strikes at the heart of how we conceptualize "mental health". The standard today would be for you to be diagnosed with "Gender Identity Disorder" under the American Psychiatric Association's Diagnostic and Statistical Manual - 5. The DSM is the textbook that lists, revision after revision, the available categories of diagnosis and diagnoses proper. The DSM changes the conversation around mental health - for better and worse. It shapes and language we have available to us to converse about gender and mental health - first and foremost by labeling gender variance as a disorder. This may be merely a formality, but nonetheless, the highest order of the land is to label this a disorder that needs correction. The disorder title is still utilized whether or not WPATH standards are appreciated and transition is determined to be an integral portion of a treatment plan.

The "disorder" label tells us what it is - even if we don't agree with the disorder label. Being told you have these feelings because your trans is similar to being told your trans because you have these feelings. You're not asking what these feelings mean, you're asking why. The "why" is essentially a question about your own life context. We know that dysphoria presents as a wide spectrum. We know the dysphoria ranges from the kind of narrative of "I've been born in the wrong body and always known something was wrong" (which blends with the disorder narrative) to the "I'd be more authentic, more comfortable, with a different way of being in the world" (which blends more with a radical-sociological narrative).

I'd advocate that you both take this question up in therapy to connect your life context to your current situation while also tolerating the difficult ambiguity that comes with choice. Looking, for example, at potential gender trauma, doesn't negate the absolute beauty of your desire. Looking at your personal history to make better sense of how you've come to this point doesn't negate the simultaneous need for your transition if you choose. You can be both always-have-been a beautiful girl/woman and engage in a process of becoming. The infinite love of transness is it's opportunity.