Clara's eyes light up as she takes in the stars above her, awe and rapture pulling the corners of her lips up into a wide smile. An upturned face tilts towards the Doctor and he wonders if she can hear the steadily increasing speed of his double heartbeat.
He knows he ought to be looking at the wondrous nebula above them but he finds himself transfixed by the small figure sitting next to him, legs swinging across space as they perch on the step at the front of the TARDIS. Never in his considerably long lifetime did he think he would see something so human to rival the beauty of ancient, perfect constellations. But he has; and she's real and solid and he's not quite sure what he's feeling when he looks at her.
And he simply cannot tear his gaze away; because she is so beautiful to him. Though some people might not see past the shortness (cute) and the upturned nose (charming), he sees straight through those things and his gaze falls onto her soul; how brave she is, and clever. Sassy and funny and perceptive. Curious, and loyal. He sees all of the characteristics that make her who she is and it is these things that make her beautiful to him, not just the physical aspects of her.
Of course, he worries that he might lose her; worries all the time, in fact. Grasps her palm tightly and hugs her and cups her cheek in his hand just to remind himself that she is still there, real, solid, not an illusion or a dream. He cannot bear the thought of losing her again; not this Clara.
Not after all their adventures and beautiful memories, not after all the smiles they've shared and cups of tea they've sipped together. He can barely imagine how painful it would be to lose her and reunite only to find that she has no recollection of him at all.
So he protects her to the best of his ability; asks if she's all right far too often and tries not to let her out of his sight. He is scared that one day, it won't be enough; that he'll let something happen to her and lose her again.
Of course, Clara doesn't understand this worry - how could she? She doesn't know how important she is to him.
And maybe one day, he thinks, he'll be brave enough to tell her how he feels. How he, the twelve-hundred year old Time Lord, is in love with a human.
But for now, he picks up her hand and intertwines their fingers and pulls her closer to him, feeling her single heartbeat and her head again his shoulder, in this moment of quiet harmony they share, and he is content with that.