Analysis
Stella at a Glance
Stella is introduced in the beginning of the play as "of a background quite different from her husband's." We can see that she is much more reserved and meek compared to Stanley. She adopted this quiet behavior because of Blanche's talkative tendencies, as seen in Scene 1.
"You never did give me a chance to say much, Blanche. So I just got in the habit of being quiet around you."
Stella also has a very deep love for Stanley, willing to forgive his outbursts even if she were to be hurt, and continues to stay with him even if she disapproves of his actions.
A Deeper Dive
Why Does Stella Choose Stanley, Always
This play takes place in New Orleans, Louisiana in the 1940s. Society and life back then is very different compared to society nowadays. A woman's expectations then included being a good housewife, quiet and submissive to her husband, taking care of the house, and bearing children. This accurately describes Stella's role in her relationship with Stanley--the ideal housewife. Men on the other hand, are expected to be the breadwinners and financially support their family. Women rely on their husbands to care for them financially due to them not having their own source of income. This context is extremely important to understand why Stella always decides to stay with Stanley despite his behavior.
The first major negative interaction we see between Stella and Stanley is in Scene 3, where Stanley gets mad during poker night and attacks Stella. We can see from the reactions of the other characters, it's a very serious situation, and Eunice threatens to call the police if he won't leave Stella alone. Yet, by the end of the scene, Stella goes back to Stanley, much to the surprise of Blanche. This behavior, in a modern context, is obviously not okay, however one must consider the context of the time period.
Unfortunately, domestic abuse was normalized and expected in households, and this is further reinforced in Scene 5 when Eunice and Steve get into a fight and Stella laughs a bit.
Furthermore, in Scene 4, when Blanche is listing the brutal, animalistic behavior of Stanley, Stella chooses to feign ignorance to Blanche's words and immediately goes to hug Stanley when he arrives. And, most importantly, Stella's explanation for sending Blanche away to a mental institution.
"I couldn't believe her story and go on living with Stanley." Page 1041, Scene 11, line 36.
We can assume that Blanche told Stella about what Stanley did to her in Scene 10. Stella hears this and chooses to continue staying with Stanley. This decision absolutely tears her apart, as she is seen sobbing throughout the entire ordeal as Blanche is taken away. It's likely that she's torn about her decision, because despite Stella's love for Blanche, she's unable to genuinely get her the help she needs.
Firstly, it's the 1940s, so it's very unlikely that there'd be any resources to help Blanche with her mental issues. Secondly, if she were to leave Stanley in order to protect Blanche, she'd have no money or assets or anything of her own to support them with, especially since she has a baby she needs to take care of. It's likely that Stella chooses to stay with Stanley less out of love for him, and more out of the realization that if she were to leave, there'd be no way for her to support herself.