You have all the elements, but it's not quite clicking together yet. The weather is lovely, you have shade from a lush tree, the barn is cool, and your gf is attractive.
Her pose seems somewhat closed and defensive, no two ways about it. The arms crossed and pressed in is somewhere between "I'm chilly" and "you're a creep."
On a technical note, the color balance seems to lean too far into the green. You should adjust her skin tone... Just that might go a long way to making it a little more inviting.
Compositionally, I wish it had more of that branch from above, an a little more of her as well. Part of the problem might simply be that you're shooting down at her. Don't be lazy, drop a knee and shoot her from her shoulder level, or possibly even belly-button. It's more flattering. It would also allow you to get more of the barn and less of the floor... and more of the branch I crave for.
I think the DOF is perfect. She is isolated from the background, but it is possible to recognize the features and state of the building.
...and a fill light from camera left might have been nice, and possibly a reflector front-right-low, but that's just my current obession getting the best of me ;-)