The main characteristic that differentiates the character of Blanche Dubois from the character of Stanley Kowalski in the play A Streetcar Named Desire is their upbringing. Their diverse and complex origins played an important role in how they developed as adults. Ultimately, it would make their differences so remarkable that they ended up hating each other until the final tragedy occurred.
Blanche and Stella Dubois grew up as Southern "aristocrats". Their family had a solid financial foundation and none of the two girls had to work for much. They were maybe even spoiled, and always waiting to get what they wanted. Before the fortunes of the Dubois family came spiraling down, it was Blanche who had to face the situation. As a woman with little knowledge of poverty and struggle, Blanche had to come up with ways to survive in a society that she did not understand. She lost everything: Her dignity, her self-control, her reputation, and even her place in society, due to the myriad of bad choices she made with sex and alcohol. Ultimately, she chose to become the embodiment of the very dignity that she lost after she became a nobody. Hence, she would walk around pretending to be a snob, over-dressing, hiding the horrid things about her past, and still maintaining that she had rich friends to help her out.
These very qualities were what made Stanley Kowalski detest Blanche so much. Stanley was a blue collar, former military man with a no-nonsense attitude towards life. He was rough, dingy, chauvinistic, and misogynistic. Also, he was a loud and obnoxious drinker and gambler. He had no concern for the needs of his wife or any other woman, and he treated women however he pleased. He beat up his wife whenever she made him angry, even when she was pregnant. He demanded to be respected as the "man of the house", and the presence of Blanche threatened him. In the end, he discovered that the very judgmental and snobby Blanche had nothing to be so proud of. He then showed Blanche how he could have control over her by exposing everything that he found out about her. At the very end of the story he rapes Blanche, leaving her so traumatized that she ends up in a sanatorium. Stella did not care.
Hence, the origins of Blanche and Stanley were so diverse and so complex in their own right that none of them could have ever crossed paths in life without a catastrophe.