I have no idea what mz dirtbiking is but if it is the only thing in common at the beginning then it's got to be where they first meet right? If they heard rumors about one o each other it would help develop feelings of see and wonder before they truly meet face to face. If not, then teenagers can fall in amor at the drop of a pin. I assume mz dirtbiking involves danger and tricks. Just have the girl do some awesomely impressive stunts that get the dudes attention then have her brush him off at first making him desire this mysterious chick who rocks his dirt bike world.