Spencer Pratt actually has regrets –- he says he’s sorry for calling Mary-Kate Olsen a "troll" –- but it’s because MK brought up a very sore subject … soccer.

Who knew? Us reports that Pratt says the celebutwin brought up an "emotional subject" when she told a story on Letterman about his throwing a hissy fit on the soccer field during high school. In fact, Pratt reveals that he’s "secretly jealous" of MK because he wants his face "on a lunchbox" just like her.

The pair have been feuding ever since he supposedly sold shots of a tipsy-looking MK to a tabloid.