Why is she being so insistently stubborn on remaining dull and alone? It's not like she'll die
Stuff like this is why these stories need a hundred chapters to gain any romantic progress
Two reasons, really - one meta, one just depressing reality.
The meta reason is already answered by your question (and I'm sure you're aware of that) - it keeps the story going. And not just in the "let's milk this serialisation for as long as we can" sense, though it works out that way far too often . . . Broadly speaking, stories need some form of conflict, and some form of progression towards resolution of that conflict - things need to be bad in some way, and they need to
change somehow as a result of the events in the story; in a story focused on an introverted and unhappy character, that progression normally comes from that character becoming less introverted and more happy. But if you've set your story up that way, as soon as they
do become happy the story is basically done, so you need to find some way to keep them unhappy while you tell the story.
A story feeling like it's dragging on painfully is generally driven by a mismatch between the way the central conflict is framed, and how the progression towards resolution of that conflict is paced. If your story is focused on the unhappy character and the progression of that character is towards happiness, then the major arc of your story is tightly constrained by the gap between the focus character's unhappy state and their happy state, along with the things that drive their unhappy state. A small gap can make a nice short story; a massive and literally insurmountable gap can be the foundation for a tragic saga; whatever the case, the pacing of that progression is key. This is complicated by differences in the way people judge the significance of the gap and the drivers for it - particularly for stories focused on interpersonal relationships on a small scale, one person's massive critical life-changing issue is another person's meh (smaller, more tightly focused stories that happen to focus on unusual conflicts are often relegated to niche audiences because of that). Regardless, mismanaging the progress towards resolution of the core conflict is probably the biggest failing in storytelling, and avoiding it is one of the hardest parts of creating stories (the hardest by far being naming your characters, of course).
The depressing reality reason is just plain depressing, though: people with mental health issues (which is very much the case with this MC) pretty much
always find it extraordinarily difficult to change their way of thinking. That's pretty much the definition of mental health issues, after all - modes of thinking that are harmful to you (and potentially those around you), which you have little to no control over. Most storytelling will avoid dealing with this kind of thing realistically, because it totally fucks up the progression of the story - how can you tell a satisfying story about a character who's constantly taking one or two steps forward only to slide back into their hole again, often for no readily visible reason?
It's hard, but when it's done well it can be extremely rewarding, and it's something that's
important to see, even in stories where it's not the main focus (which would normally be the case for something like this) - people who
are trapped by their mental health issues are often in that position because they
can't see any way out; seeing a realistic portrayal of characters they can identify with actually being able to move forward and find a better life can make a big difference.
We're currently only two chapters into the story, so it's hard to know how this will progress. It's piqued my interest, though, and the hints of darkness (the falling body from the first chapter, for example) make me hopeful that it will take the harder but more interesting and rewarding path . . .