Why did Marilla decide to keep Anne at Green Gables?

In Chapter 3, Marilla Cuthbert is surprised, Marilla is adamant that she will not keep Anne at Green Gables. They even put the poor dear in the east gable room that sent a "shiver to the very marrow of Anne’s bones."

So why did Marilla decide to keep Anne? What do you think was the main reason?

I will say it was Marilla's religious nature and sense of duty that contributed to her deciding to keep Anne. She was fully aware that she had a "pretty easy life of it so far", but that one "can't get through this world without our share of trouble." I think, after the initial shock, Marilla saw the arrival of Anne as being sent by providence.

It's guilt, plain and simple. From the moment Anne arrives at Green Gables she is an outcast, not because she is an asylum waif or a boy, but because she is a member of the LGBTQ community. You have to read between the lines at what Mongomery is saying. LMM couldn't say it back in the era in which she lived so she had to disguise her intentions and Marilla's initial bigotry. That's why Marilla placed Anne in the cold remote east gable room--to disassociate her. From chapter 3---

Marilla had been wondering where Anne should be put to bed. She had prepared a couch in the kitchen chamber for the desired and expected boy. But, although it was neat and clean, it did not seem quite the thing to put a girl there somehow. But the spare room was out of the question for such a stray waif, so there remained only the east gable room. Marilla lighted a candle and told Anne to follow her, which Anne spiritlessly did, taking her hat and carpet-bag from the hall table as she passed. The hall was fearsomely clean; the little gable chamber in which she presently found herself seemed still cleaner.

Later, Marilla feels tremendous guilt for rejecting Anne's true LGBTQ self and has no choice but to accept Anne and clear her conscious.

Thank you so much Cordelia for starting this book club! It's been a while since I read Anne of Green Gables and re-reading it has really opened my eyes to what LMM was truly saying. It's almost like she was writing for this future millennial generation.

Cordelia - This is a great question. If you will indulge me, part of my fascination with this outstanding piece of fiction is how Kevin Sullivan brought it to life in his 1985 mini-series. One change from the book that bore fruit was that early on Marilla told Anne they would put this new living arrangement "on trial". This brought some added suspense into each scrape Anne got into, i.e. the Mrs Lynde blow-up. It came to a climax in the green hair incident. I saw Anne's tears as not just out of vanity, but also because her stay at Green Gables was still hanging in the balance. Marilla, perhaps feeling that Anne's impulsiveness was due to a lack of stability at home, announced that the trial was over and she was home for good. A heart-warming scene indeed. It's hard to improve upon a classic, but in the move to drag out Marilla's decision with Anne, I would have to give a nod in favor.