As human beings, our biological impetus is to procreate, irrespective of how technologically advanced and progressive our society becomes.
When two people who are biologically designed to procreate are intent on building a platonic friendship, that biological instinct is an impeding factor.
When a man and woman have such a close relationship, that it is akin to that of siblings, it means that the two will naturally care for each other deeply.
For a good while, you cared about her like you would a sister, and this made sense.
Nonetheless, unbeknownst to you, the closer you got to her as a friend, there was an increased potential that you would develop romantic feelings for her.
To date someone who cares about you but is emotionally unavailable is to put yourself at emotional risk.
Being that this is someone that cares about you, you mislead yourself into believing it is worth it to date her.
When you do this, you conveniently put the fact that she is emotionally unavailable to the back of your mind.
This is where you are demonstrating that you want to focus on the positive elements of your relationship with her.
It makes you feel good to believe that she cares about you, and that in time, she is going to care about you all that much more and consequently become emotionally available to you.