It was really obvious based on promo pictures in season 1 the original intention was a big time triangle between Nathan/Peyton/Lucas:

You can just tell that's where it was going initially. Then Mark probably put together Nathan/Haley to cause more trouble between Lucas and Nathan. He went away from Nathan/Peyton/Lucas which is fine by me because I did not like the abusive nature (neglect, emotional type of abuse) Nathan was giving Peyton. It made my skin crawl. I couldn't imagine a character like Peyton putting up with and even still wanting to be with Nathan. It was clear she was over him very very early season 1 and it made sense for her character TO feel that way.