Undoubtedly pride is one of the most obvious reason.
But that's not the only one.
It's important to notice the period when the story takes place : the eighteenth century.
A nobleman would have been seen as impure or insane if he had married a poor girl with such a low connections.
It's evident that Austen wants to highlight the struggle between "reason" and "feelings".
Darcy uses reason when he tends to avoid her or not speak to her.
Only afterwards he realizes that it's useless to put up a fight against "The Heart".
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