Emily is plagued by a diversity of factors that render her prone to an unsuitable connection with the rest of the world. As with everything, it all starts at birth and continues throughout childhood. If a series of issues are not resolved by then, adulthood is likely to perpetuate them forever. It is all the eternal debate of nature versus nurture.
Our "nature" manifests our inherited traits. However, we do not have to live with them. Throughout our upbringing, "nurture" will curb those inherited traits, fighting or operating against(or with) the environment so that they do not dictate our behavior entirely.
We know that Emily has both natural and nurtured traits that lead her to behave in an eccentric way.
NATURE: People in our town, remembering how old lady Wyatt, her great-aunt, had gone completely crazy at last, believed that the Griersons held themselves a little too high for what they really were. [..]
NURTURE: We had long thought of them as a tableau, Miss Emily a slender figure in white in the background, her father a spraddled silhouette in the foreground, his back to her and clutching a horsewhip, the two of them framed by the back-flung front door.
This shows that there is inherited insanity in the family (nature) and that these behaviors were reinforced by an over-controlled upbringing by her father (nurture).
This can safely win the argument that these are the strongest factors that rendered her prone to act in total disconnect from the world; her proneness to insanity, connected to a poor upbringing, left her unable to connect (and get rid of) people in a proper manner.
The proof of this is that Emily's acts of insanity, both temporary and permanent, have to do with her inability to let go.
The first time it was her father, whose death she chose to deny to herself, and whose body she refused to give up.
She told them that her father was not dead. She did that for three days, with the ministers calling on her, and the doctors, trying to persuade her to let them dispose of the body. Just as they were about to resort to law and force, she broke down, and they buried her father quickly.
Here, her insanity was the reactive attachment to her father, whose presence is profoundly influential to her. She is not able to let go, and yet she is able to live with his dead body for three days. Insane.
The second, more permanent, show of insanity was the inability to let go of Homer Barron, the drifter with whom she apparently loves in love and decides to show herself with around town. Once the man goes away, he is seen coming back for a while, not to ever be seen again.
as we had expected all along, within three days Homer Barron was back in town. A neighbor saw the Negro man admit him at the kitchen door at dusk one evening. And that was the last we saw of Homer Barron. And of Miss Emily for some time.
Later on it is found that Emily had presumably killed Homer using rat poison that she purchased, and then lived with the dead body until she herself was dead. Emily's ability to withstand dead bodies is clearly insane. Moreover, her inability to help herself to either fit in or control her behavior just make her problems worse, ending in the inevitable.