Darcy and Bingley are so different, I can't help but wonder how they became friends. I think it's unlikely they met at university, perhaps at school when they were younger? Bingley is from the "north of England" and Derbyshire is fairly north, perhaps they were neighbors?

But their tempraments are so different and I found this to be interesting:

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, and ductility of his temper, though no disposition could offer a greater contrast to his own, and though with his own he never appeared dissatisfied. (ch 4)

So Darcy admires, in part, Bingley's "easiness and openness" of temper. I was struck then by Darcy's thoughts on Elizabeth in ch. 6:

...in spite of his asserting that her manners were not those of the fashionable world, he was caught by their easy playfulness.