What is the reason for Darcy and Bingley's "very steady friendship.?"

They are total opposites....... opposites make the best of friends because they're constantly offsetting each other and there's never a question of offense when advice is required. m

"Between him and Darcy there was a very steady friendship, in spite of a great opposition of character.—Bingley was endeared to Darcy by the easiness, openness, ductility of his temper, though no disposition could offer a greater contrast to his own, and though with his own he never appeared dissatisfied. On the strength of Darcy's regard Bingley had the firmest reliance, and of his judgment the highest opinion. In understanding, Darcy was the superior. Bingley was by no means deficient, but Darcy was clever. He was at the same time haughty, reserved, and fastidious, and his manners, though well bred, were not inviting. In that respect his friend had greatly the advantage. Bingley was sure of being liked wherever he appeared; Darcy was continually giving offence."