The ending pays homage to its fairy-tale roots in a beautiful way — with a little touch of humour that says so much about the story in general and all of the beautiful complicated relationships involved.
Howl: Now, I think we should all live happily ever after. It should be hair-raising.
Calcifer promises that Howl will only be genuinely in love if he leaves the house without spending two hours in front of the mirror. At the climax, Sophie is trapped in the Witch's castle. Howl arrives just in time to save her.. looking completely disheveled and messy.
After Howl's Disney Death, he only takes a few seconds to get his bearings...and then tries to get up, saying he has to rescue Sophie, clearly caring more about her than himself.
Markl asking Sophie if she's going to leave, after her mother stops to visit.
Markl: That lady said she wanted you to live with her now. Sophie: Yes, well... at least she cared enough to visit. Markl: Don't leave, Sophie! *Hugs her apron* I love you. You have to stay! Sophie: Ah. I love you too, Markl. I'll stay.
The scene where Sophie begs Howl not to go and fight. What Howl tells her afterwards is "Dawwww" worthy.
Sophie: Let's run! Don't fight them, Howl! Howl: Sorry, I've had enough of running away, Sophie. Now I've got something I want to protect. It's you.
Near the end we discover Heen was supposed to spy on Howl and his friends. Turns out he grew to like them (mainly Sophie) too much to do that. He laughed at Madam Suliman when he finally calls her!
Turnip Head shielding Sophie from the rain as she's crying her heart out due to Howl's selfishness. When she realizes he's there, she immediately dries her eyes and remarks on how kind he is.
Crowning Soundtrack of Heartwarming: the first few bars of "The Boy Who Drank Stars" doubles as this and a Tear Jerker.
When Sophie first turns into an old lady she crosses a bridge, coughing from the train smoke. A random man just standing there offers to help her. "That was very kind," Sophie even says.
When Howl has returned Sophie and Heen to the platform where the Witch of the Waste has Calcifer in her possession, she says that Howl needs his heart back. The Witch tries to feign ignorance, and Sophie answers this with perfect understanding and earnestness, embracing the witch and pleading for the heart. The show of true love convinces the Witch to return it.
The ending itself is uplifting overall. Sophie and Howl have become stronger people and are able to properly express their love for one another, Turnip Head regains his true form, and he and Madame Suliman set about putting an end to the war that's been plaguing the country for the entire movie. We get the last scene, with Howl, Sophie, Markl, Calcifer, Heen and the Witch of the Waste all living happily on their now-flying castle. Awww!
The scene where Howl and Sophie first meet, him rescuing her from the Witch of the Waste's guards, culminating in the famous Flight of Romance on the rooftops. The whole thing is just so beautiful and romantic, and is almost guaranteed to get you shipping them.