First the causes of transsexualism. It's not proven but indications are we have the wrong gender programmed into our brain before birth due to hormone exposure. You can read more about it here.
https://en.wikipedia.org/wiki/Causes_of_transsexualismWe are not cookie cutter images of each other and experience different emotions about this in my case, it was my role in life that make me uncomfortable. A example to you would be think of waking up each day and having to live life as a woman instead of a man. Others feel uncomfortable with parts of their body such as a FTM being uncomfortable with their breast. Pretty common is us is we understand what the sex hormones do to us is something we don't like and we almost carve the sex hormones of the opposite gender.
We all feel for what ever reason that we were born in the wrong body and want to do something to bring our body closer to matching our brain.
While all of us were born this way, some know as early as 3 years of age what they are. Many like me figure it out when we enter puberty. A few discover it much latter in life but there are many who know in puberty and suppress it until much later in life. It mostly a mater of when we are willing to admit it to ourself.
As we are not cookie cutter, our treatment isn't either. Many like me go all the way and have surgery. Others find some compromise that make them comfortable without surgery. In my case, had surgery not been available, I could have survived by cross living because living as a woman filled enough emotional needs. Don't get me wrong, I did want the surgery and I never have regretted it. If you are on this board long enough you will find a number of poster who for one reason or another are comfortable without surgery. I consider them to be my equals but their needs were a bit different than mine.
The standard lecture. All people have two things and they are independent from each other
1. Gender identity is who you want to go to bed as
2. Sexual Preference is who you want to go to bed with.
We are no different from others and we can be heterosexual, Homosexual, Bisexual and Asexual before or after surgery. There are a few other you will learn about if you hang around here but those are the main ones. One difference is that in coming to terms with our gender identity, we often come to terms with our sexual preference and find we are more flexible that we thought. An example would be MTFs who were only with women before treatment may enjoy the company of men after surgery. On the other hand, it isn't uncommon for MTFs to remain with their wife after surgery. There is no telling and it is up to the person transitioning as to what their future life will look like.
Pretty much everything above applies to MTF or FTM and I see the same thing in both my sisters and brothers that I see in myself. The difference between our paths are mostly physical. In general it's harder going from MTF but the surgery is better. Going FTM, the hormones do wonderful things but the surgery isn't as hot.
I am stopping now because I think I have covered most of your questions and I don't want to overthink your questions. If I missed something or you have more questions, let me know.
Edit - I forgot something, Intersex can occur at any age and is the result of a visible genetic defect. They are a very special case and have to be assumed as having gender identity issues unless determined otherwise. There are several here and often discuss their issues.