Because right now, she likes boys. In fact, there have been problems at school with her and another boy talking about kissing each other (they think its funny)... Last year she got in trouble for chasing the boys on the playground saying she was going to kiss them... SHE IS ONLY 7, what am I gonna do with her... UGH!!
She tends to gravitate towards the boys in general and seems to choose to play with the boys over the girls. Which may be partially because she is somewhat socially immature. She is smart as can be, but compared to other little girls her age, she is not quite as mature as they are. Considering girls mature faster than boys, maybe that's why she likes to play with the boys...