In the anime series 'Ouran High School Host Club' what's Tamaki's reasoning behind making Haruhi a host?

If she had continued on as an errand boy, the debt would have been paid off eventually... even if it took years. 

The hosts don't charge any of their customers, and even if they did... Tamaki tells Haruhi she only needs to get 100 customers for her debt to be erased, and unless they were charging a fortune 100 customers wouldn't even come close to paying off her debt. 

So, what's in this deal for Tamaki? At the time of the 100 customer arrangement he's not attracted to her, he still thinks she's a boy. So it can't be that he just wants to keep her around. 

Is it a personal challenge for Tamaki to see if he can train the "commoner" well enough to get 100 customers? 

What's his reasoning?