I think guys by nature tend to really like competition.
My husband doesn't need to lose weight but he always is trying to gain weight and bulk up (which can be oh so frustrating). So needless to say we have different issues but one thing we do together to motivate us both is hold gym competitions. We keep a tally on our calendar at home to see who can go to the gym the most in one month. The loser buys the winner something he or she wants at the end of the month! It has worked really well for both of us!