Well I see some common possibilities, which I'll categorize into "she would" and "she would not". This'll be quick and simple.
She would: she doesn't think she's ready to date or she sees something as an obstacle in her life, either currently or in the future; she is dating someone else, or has had someone in her sights for some time (the way I hear, it's most common for INFJs to have tunnel vision in romance)
She would not: she feels like her role is somewhat motherly in your relationship, and although she likes it that you can be relaxed with her(?), you two don't necessarily make a romance right now; you could be confused about your feelings, and it might be more feeling accepted than love (I always crushed on female therapists as a teen), or at least she might see it that way; she greatly values her solitude and peace, and doesn't feel like she would want to be in a romantic relationship with anyone.
Anyway, it is true that your only realistic course of action is to let it be, and the course of feeling will of course govern itself, but that's okay. It won't change a thing, no matter how many possibilities we might think of. I'm simply glad you have someone with whom you feel accepted as a human being.