This image will teach me to not write up crits on good photos quickly! The community has and is speaking, and I owe it a critique - no pressure!
The comments on the image have already captured a lot of why it works, but they bear repeating.
The first thing that struck me were the pastel colours, and the sense of peace and quietness - maybe sadness, following the title - that this image has. My personal preference is normally for more saturated colours, but this level is just right, and is essential to creating the feeling of the image. You have managed the backlight highlights without losing any detail in the shadows - really nicely done.
The effective use of shallow depth of field, and the inclusion of the grass build a feeling of privacy. The viewer is distanced a little from what is going on - that this is a private moment - maybe of sadness (but possibly other feelings too?) The long focal length has worked well for this.
The model and setting work together really well. Beautiful features and pose, but relaxed and natural looking, with a pensiveness about her that almost makes the picture feel intrusive - if it weren't for the sense of distance created by the foreground grass. Part of what works is the way much of the grass is leaning to the left with her - the whole image comes back to the direction of her gaze - very effective.
A beautifully understated, but powerful image, and I don't think its fav count is finished growing! Well done!