"The Runaway Bride" is such an iconic episode, really. It was just Donna in her wedding dress appearing in the TARDIS while the Doctor was in the middle of a heartbreak, and she immediately changed his life, and hers in the process, from the second she started yelling at him to TAKE HER TO THE CHURCH.
This woman came up with a whole variety of pet names for the Doctor in a span of a few hours, some of which (SPACEMAN) would become legendary and character defining and would be remembered fondly by the Doctor.
This episode is full of iconic scenes, like Donna getting kidnapped by a robot Santa and the Doctor trying to save her by encouraging her to jump out of a moving car on a motorway, and while Donna is refusing to jump arguing that she is in her wedding dress, the Doctor's best response is to compliment her look like "girl, you look perfect for a jump out of a moving car on a motorway."
And the next scene on a rooftop solidifies the silly tone of this relationship by the Doctor putting a biodamper ring on Donna's finger cracking a wedding joke, which Donna immediately joins in on.
This scene right there was the start of their friendship and that bond that would last for the rest of their lives. It all started with a robot Santa, a wedding ring and a silly joke which hilariously continued into the next scene with the Doctor and Donna standing under the "Just Married" banner. Comedy gold.
A very underrated moment of the Doctor continuing the affectionate insults tradition between them by calling Donna a 4H pencil, trying to explain to her how she got transported into the TARDIS.
There were these small moments throughout the episode that showed exactly how this friendship of a lifetime started. Like the Doctor taking Donna to see the creation of Earth right after the moment her fiance betrays her and admits he hates and wants to kill her. The Doctor sees all that and tries to distract her by showing her something so extraordinary and beautiful that no other human saw before her. And it works. This woman who has been so unimpressed with him calling him SPACEMAN (derogatory), was now cracking jokes and laughing with the Doctor and very much enjoying his company.
The Doctor and Donna who spent the good portion of the episode trying to get rid of each other, were now ride-or-die besties.
The Spaceman and Earthgirl are now imprinted on each other so much that this little Christmas adventure will become a life changing event and a start of a relationship so meaningful and deep that it will end up changing the Doctor's and Donna's lives fundamentally and save this world and a multitude of others.
The Doctor started the episode with "I don't want you here anyway" and ended it trying to make Donna laugh and making it snow when he heard she didn't like Christmas, and openly telling her he would be lucky to see her again and inviting Donna to come with him.
And Donna who starts the episode yelling at the Doctor and being so unimpressed with him, ends up understanding him on such a deep level and leaving him with the words that he would carry for regenerations to come. And failure to remember these words would lead to the death of this incarnation.
Donna is asking the Doctor to find someone, and she doesn't yet realise that he already found that someone he needs, a platonic soulmate, a friend, a partner, and that someone is her.
"The Runaway Bride" was a start of a many years (for Donna) and many centuries (for the Doctor) road from that rooftop to the garden where they will sit together with a bright shared future ahead. Best mates having the best of times for the rest of their lives. TOGETHER.
