There can be any damn reason which make your mood upset or sad. Maybe you had a bitter quarrel with love family or friend... Or it might be a Break up, or one of your dreams being shattered without you being constant trying... HeY! but wait sadness is just a part and not life itself... Love is a remedy to being sad. Divert your mind. Stay with people you love and who care for you... You will feel great after that. Now it's not an instant remedy for you bad mood but is to the best of extent the most probable solution. So love every moment as it is yours...