By the time a little girl reaches age two, the process and ritual of making friends is well underway. Even in its most infantile state, socialization among others is of utmost importance in our development as human beings. When you’re that little it doesn’t matter who you play with, girls or boys, it’s all just fun.
But by the ripe old age of four-or-five-years-old, we begin gravitating to our same gender for friendship. It’s because we inevitability have more in common, and usually don’t get ridiculed for doing “girl things” by other girls (at least at that age).