Exercise is contagious.  The more you do, the more you want to do.  So too, the less you do, the less you want to do.  This is because it takes energy to exercise - or even to get yourself out of your comfy bed to do it!  But exercise also releases endorphins in your brain - the chemical that makes you happy and feel accomplished.  If endorphins are released on a regular basis, your brain will get addicted to this "exercise high" the same way it gets addicted to drugs.  You'll be finding excuses to exercise instead of not to.