People have said many times how Fourteenth Doctor is so much more open with feelings and emotions than Ten. But it is especially huge when it comes to Donna specifically.
Ten was so reserved and held back so much when it came to expressing feelings and voicing them, that his companions never fully understood how much they meant to him. Ten was very flawed and very HUMAN and while he had so much love in him, he was equally afraid to express it, show it unabashedly.
Did Donna ever truly understand how much he loved her? Did he ever tell her how much she meant to him on screen? When Ten knew he was losing Donna, and she was about to disappear in front of him, he knew this was his last moment with her, and the best he could do while she was conscious is hold her at arm’s length and tell her “We had the best of times”. You could see he was dying inside and this moment is killing him but he NEVER SHOWS it to her. While it’s all in his eyes, Ten still doesn’t tell her anything that would have an emotional weight or reveal the whole depth of his feelings.
Ten kept all his feelings bottled so deep inside, that when they finally started to bleed out, he made the worst mistakes possible. It ended up killing him. And the breaking point was losing Donna. He carried so much guilt over the way he lost her and the way he let her, his best friend in the whole wide universe, down, that it echoed over to the next regenerations, in Eleven’s guilt and Twelve’s face.
That scene in the TARDIS in “Let’s Kill Hitler” where Eleven is poisoned, and the TARDIS is showing him images of former companions when Eleven asks “Give me guilt”. The TARDIS shows him Martha first, and there is so much to be guilty about when it comes to Martha, and then Eleven asks for “also guilt”, and the TARDIS shows him Donna, and when Eleven asks for “more guilt”, the image doesn’t change. It is still Donna, TARDIS is showing Eleven the best friend who got let down so hard by the Doctor. And the Doctor still carries that guilt and regret and heartbreak with him, because he loved his best friend and her attachment to him almost got her killed and hurt her in the worst way possible. “Stand too close, and people get burnt”, just like Martha said.
Fourteen though. He is a completely different person. He doesn’t just love Donna, he ADORES her, he is here FOR HER, this face is here BECAUSE OF HER. He tells a complete stranger he just met that he loves his best friend Donna, and repeats it twice.
When Donna honestly asks who would care about her 'cause she thinks she is just no one, Fourteen angrily cries “I DO!” because that’s his brilliant and amazing best friend who after all these years still thinks so little of herself and he hadn’t been there to remind her of that, and it is all his fault that she doesn’t remember how fantastic and special and extraordinary she has always been and how many times her brilliance saved worlds and lives, including the Doctor’s.
Fourteen is crying and screaming and breaking down, when he thinks Donna is about to die in front of him, all because he is so afraid of losing her again. Fourteen cradles Donna’s body and cries when he thinks he had lost her and tells the Meep’s soldiers to kill him, because he doesn’t care anymore, because someone he loves so dearly is dead in his arms, and it is all his fault.
When Donna gets her memories back, Fourteen almost immediately confesses how losing her KILLED HIM and how he remembers every second with her. He tearfully confesses that he has missed her for all those years they were apart, it’s been millions of years they spent apart for the Doctor. All those years the Doctor needed Donna who would make him laugh again, Donna who would keep him grounded, Donna who would call him spaceman and silently hug him when he needed it the most, Donna who would understand and be there and be his touchstone and anchor and guiding light.
Fourteen knows that she can’t stay with him forever like she used to want to, but he still gives her both of his hearts and tells her how much he cherishes their time together. He offers her coffee just the way she likes it, he remembers such small details about her after literally thousands, millions of years! And he shows it to her. Fourteen is so full of love and he is not afraid to show it, he is rushing to love, to express it before Donna is gone from his life again. And he is trying to make up for all those years as Ten when he couldn’t say these things to Donna, and she never knew just how much she meant to him. Donna never knew that she was his soulmate, that her presence made such an impact and her absence led to Ten’s death and that he died saving her grandad.
Fourteen kisses Donna’s hands and hair and hugs her like she is the most precious thing he knows. His hugs are also different from Ten’s. Fourteen wraps himself around Donna and holds her tight, this is how he shows his affection. He wants Donna to hear, know AND feel how much he cares. Because he already knows what it’s like to lose her and he wants her to know in case he has to let her go forever again.
When there is a threat of the world ending and his wonderful selfless Donna once again stresses that it is not about her, the Doctor solemnly replies that it really is. Because here right now in mortal danger he is willing to put her first and thinks of her wellbeing before anything else, because Donna Noble means the world to him.
Fourteen loves Donna so much, that he is willing to stay with her forever, staying still is not so bad when it’s with her. Fourteen is so tired and weary and bruised and exhausted but when he looks into Donna’s eyes, he doesn’t feel so tired anymore.
Fourteen got Ten’s face because he needed rest, but also maybe, just maybe, because Ten didn’t want to go so badly that he broke through and got the chance to come back so he could have another chance, to have a life and be with his best friend again and come home. Fourteen was born out of love that Ten had for Donna. And that love (and the TARDIS) brought him right to Donna’s doorstep, so he can hear her call him spaceman again and have the best of times for as long as they live and have a laugh with his family in their shared garden and find out what it’s like to be happy. Fourteen gives his love so openly and freely and he gets to receive it too, he gets to bask in it and revel in it. He finally gets the reward Ten was dreaming about.
