Alright, so I'm occasionally a romance author, and one thing I've always pinned down is "What do these characters offer to each other that nobody else can?" as the most important question in selling the 'ship.
Around the time Bubbles and Faye got introduced to each other, I genuinely disliked both of them. I had thought Jeph had played Faye's cards too hard when she hit rock bottom, and ultimately forgot to remind the audience that she had sympathetic character traits. I just really couldn't find myself liking her anymore, at all. Likewise, when Bubbles was introduced into the comic, I thought she was played up too aggressively as well. Didn't actually care for her strips.
They are now, obviously, currently my two favourite characters on the day-to-day, and the storyline I most look forward to seeing.
They've been really good for each other. They have obviously made each other characters that I genuinely enjoy, brought out the best in each other, and it's been obvious that their mutual character growth has been because of each other.
As other people have said, a relationship shouldn't be seen as the natural evolution of a friendship. I agree. However, as a story that leads up to it becoming a relationship, this is pretty much the absolute gold standard on how to make it good and meaningful for the characters in it. It makes it clear why these two want a relationship with each other as opposed to the standard explaining why they want a relationship, with each other.