Gratitude Is A Choice
In the same way that you can choose to be optimistic or pessimistic, you can also choose whether or not you are going to be consciously grateful for your life.
When you practice gratitude, you're actively choosing to have a more positive perspective on life. When you learn to focus on things you have to be grateful for, you can gain greater happiness and experience a better quality of life. All it takes is you actively changing your outlook.
Some days, it will be hard to choose gratitude, but those are the times when you most need it. On 'those days', when you're feeling sad, hopeless, painful, blue, you're frequency is pretty low. Since the feeling of gratitude has a high vibrational frequency, it will raise your vibration on those 'bad days' and help you rise above those negative emotions.