Donatello could be very devoted and romantic under that purple mask and those modified glasses, and you knew it better than anyone else.
After the failed sentimental odyssey with April, he met you during a particularly hard battle, you met, when he sought refuge in your house thinking it was uninhabited. Even now Donatello is grateful for having made that rash choice.
A whole year has passed since then, and none of the gifts he gave you have ever been wasted, he has never had to suffer the burning and painful humiliation of rejection again, it moves him to come to your house and notice that all his gifts are displayed in the house and treated with the utmost care, when he was used to seeing his gifts and his heart cruelly rejected and, even worse, mocked without seriousness. For him, you are perfect.
Even his family appreciates your respectful and kind nature, which makes Donnie even happier and more reassured that he made the right choice in choosing you to invest time and feelings, in a relationship based on sincere love, trust and respect, his dream. Even to his family it is obvious and almost nauseating how much Donnie was in love with you.
Seeing you lying on that bed, however, did not make him happy, it made him anxiously worried, as well as furious for what had happened to you, he should have prevented it, not only taken care of you. Instead now...You were lying in front of him, in all your fragility, with your throat dangerously and painfully compressed and a head injury.