BH weren't friends in Season 1, and like I've said in the BH thread, that is what made sense at the time. It would just be unrealistic that BH would click automatically given how different their characters were.
In Season 2, they weren't so much good friends but they hung out in the same crowd with P, L and N.
And in Season 3 the friendship developed and obviously they became very close, I would consider Brooke to be Haley's closest girlfriend now, especially after Brooke was chosen as her maid of honor AND Jamie's godmother, two roles you don't give to just any old friend.
Bottom line, I've always loved the BH relationship and am sooooo excited for S7 and them.