If a dog misses someone they love they show it in many ways, like search for the person everywhere there is even a slight scent of them, they sit, stand or lay by the door and cry, moan or pout and sometime they may even destroy things that belong to that person or the person who is with them all the time.. It may be a simple case, in this matter, that if you play with him more on the weekends than your girlfriend does the rest of the week, that he is just bored and needs more physical activity, especially when you are not around.