I think what a lot of people are missing in the whole "standing in the rain under a canopy" trope is the setting in which it occurred. This is not simply an actually fairly common love trope of humans with everything being fluffy-sappy.
Aziraphale and Crawley, meeting after the incident in the Garden of Eden, standing on the wall and watching the first humans is literally the first time they met again after Crowley fell.
It's not like they had one nice encounter and then randomly met again and kept the vibe going with Crawley falling instantly after being given some bread crumbs of affection.
It's literally them having a nice encounter, then Crawley joining the rebellious crowd, falling from God's Grace, from Love, from Light and being literally cast out from everything he has ever known, losing his home, his job, his family, even his name. To then falling into the Darkness, into Uncertainty, no safety, no familiarity, no rules or guidelines, with a bunch of other equally traumatized beings that probably quite literally went a bit mad over this.
Basically having his world upended, the rug pulled out from under his wings and struggling to keep his head above water for a while during a period of acclimatisation. (Imagine a gay or queer teen being cast out of their very religious family of origin after coming out. It's still happening to this day!)
To then going into the Garden of Eden to spread some of his mischief, maybe not necessarily making the humans disobey God's rule, but giving them the choice to gain free will and self-agency outside of the very strict boundaries they've been born into without any choice.
Crawley does not fall in love with Aziraphale when taken under his wing due to some human trope about romanticism or that being how a love story starts.
Crawley falls in love with Aziraphale in that moment because Aziraphale is still treating him the same way, with the same kindness, the same care, affection and love that he did before. For him, nothing changed. Even though he draws a verbal line in the sand between Angels and Demons, his actions show that he is still loving enough to shelter the Demon from rain, just like Angel-Crawley did previously for him.
Crawley does not start loving him due to a small gesture. He starts loving him due to the symbolism of it after everything that happened in between.
It's the same kind of love you experience when you lost everything due to coming out and then finding out a while later that there are still people from your past for whom nothing changed, who love you the same, who will come through to you like they always have and who will be by your side no matter what.
And coming from a bunch of uber-religious angelic zealots from Heaven, that's actually quite a lot.
