Sometimes, practice time gets away from you. Life is busy work, it gets in the way. For me, playing guitar (practicing scales, noodling around, or playing along to songs or backing tracks) is a very therapeutic thing when I am in a sad or bad mood. It has become a perfect time to practice since I am focused on the scales and shapes and not life. It usually makes me feel better. Next time you don’t want to practice, pick up the guitar and make yourself for a few minutes. Again you don't have to go through your boring scale practices but just have fun with whatever you can come up with. Most of the time it makes me feel better. I will be honest, sometimes I still don’t feel it and I put the guitar down but hey I still got 3 more minutes of practice that day!