First off I want to say I think your definitely going places with your drawing. In drawing every day brings constant improvement. I am going to give you my honest opinion here. I dabble in drawing myself, but am no where near your level so take it for what its worth. I just grow tired of seeing everyone giving 5 stars for everything with out justification.
Over all this is a wonderfully clean piece. Your line work is excellent, and i really think you have managed to capture the emotion you wanted to in this piece. Now the devil is in the details over all, and besides a little variation in the hair which is nice and the highlights in the eyes, which are very nice, i find this to missing some more detail. I think some shadowing would do great for you.
Now perhaps this is more personal opinion or a stylistic choice on your part, but light and shadows are something that give depth to an image. And depth pulls you in.
Over all this is a lovely piece. Keep working hard at your art.