Well, I can tell you one thing: a possessive adjective trumps a definite article here.
In other words, the word "my" renders the article "the" unnecessary. You can't say:
"She is my the best friend." (X)
You would, however, have to say the following:
"She is the best friend I have." (√)
One way you could think of this is that possessive adjectives sort of have a definite article built right into them. For example, when you say:
"My book is on the table."
...what you are actually saying is:
"The book that is on the table belongs to me."
Possessive adjectives tend to specify a certain thing, much in the same manner that definite articles do. There is probably more that could be said about this, but that's a start.
Hope this makes sense.
Student or Learner