Kakashi had made it clear that he would not be taking on another team until one came along that met his standards.
But then Hiruzen retired and Tsunade took over the recruitment for Konoha's devision of the Karate Alliance and before Kakashi could say "I got lost on the path of life" he'd been consigned team seven, a sad bunch of teenagers intent on ruining his weekends.
To be clear, Sasuke and Naruto made the cut about two weeks before Sakura did. Kakashi is ninety-nine percent sure that Tsunade did this out of A. Sadistic enjoyment (Trying to make Sasuke and Naruto work together is equivalent to setting a lemonade stand on fire and still attempting to sell a few cups) or B. She observed the lack of collaboration between Naruto and Sasuke and realized they needed a different type of teammate. And so she chose Sakura Haruno, whose main purpose was meant to be glue but turned out to be gasoline.
Kakashi eventually decided team seven was worth a sliver of effort - so he put all the water bed mattresses rotting in his garage to good use.