Hatori's problem is that he cares too much. As a member of their dynamic trio, he brings Ayame into check regarding his outlandish style of acting out while being Shigure's confidante during times of planning (the dog and the dragon, what a pair). As the family doctor, he keeps everyone healthy and safe, especially their little angry god Akito; he probably spends more time making sure the frail child receives the best care he can give out of everyone else -- the thought of losing such a staple in his life is unfathomable. He's dealt with change once and failed, now won't contemplate losing Akito no matter what havoc the child reaps upon him.
I will take your curses with a smile, tend to your wounds as mine stay unchecked, because I must not bring harm to my patients, and that includes you.
He cares too much, and takes the pain himself. Kana -- he carries the burden of her lost memories with him every day, but it doesn't make him sad. The memories are harmful, dabbed in blood and cruel intentions, and knowing that Kana no longer remembers such times existed makes him feel both melancholy and relieved.
I will shoulder your cast-off scars in silence; for every one I take is another you no longer wear.
Hatori tells Tohru to stay away from the Sohma family, their curse will destroy her. She doesn't heed his words. She cooks and cleans and wears a smile like sunshine in spring. She's a safety blanket and a mug of hot tea in the middle of a monsoon, and Hatori realizes how badly the Sohma family needs her (probably more than Tohru needs them).
"It becomes spring! No matter how cold it is now, spring will come again! Without fail. It's strange isn't it? But it's true."
So he lets Tohru into his home, into his life, into his heart. Somehow she fits the spot by his side as easily as Kana once did. Kana who is now married and living a life of her own. Hatori watches Tohru hang the bedding to dry in the backyard and thinks how she'd make a great wife some day, to some lucky man. But not him.
"Someday, no matter how cold it is now, the snow will melt. Without fail."
He cares too much. So it explains why Hatori is right now on his knees in the dirt, bare hands in the ground, scooping out holes to plant seeds that will blossom into many different things: orchids, camellias, casablancas, so on and so on. Tohru is next to him, gently placing seeds in the holes Hatori creates. The sun is warm on their backs, but a gentle breeze wafts through in front of them, keeping them cool as they work.
Hatori remembers that winter brings spring, and with it the flowers that he and Tohru are working to create in the soil underneath them, and his heart becomes a little clearer. At that moment, the idea of being embraced without complication becomes more believable.
Tohru turns and gives Hatori a smile, and he knows exactly who he will be embracing.