@erinrew Cute concept! I think it's ok to leave the background empty but definitely add some other element of life to it. Maybe 2 small squirrels or birds (I personally like having characters in an illustration).
I second Nicole's feedback on making the trees not look similar. I can see you've tried some variation, but maybe push it a little more? Mainly right now they're the same width, height and the branching also starts at the same height.
I like the idea of the branches making the heart shape. It could be made a little more prominent.
It also depends how realistic you want to create your piece. You can exaggerate the twining of the trees further and actually have the branches wrap around each other.
My main feedback is the background color. It's too warm for me and almost looks fire-y. I'd add more purples/pinks and tone down the orange-yellow. The white bark would pop more against them too.
Even if you draw faces in the trees, how about having them face each other? I can see they still have a nice pleasant expression but they're book looking away so a part of the "togetherness" feeling is slipping away.