Everyday, no matter what, I aim to be as positive as possible. Honestly, I don't believe you will go far in life without the fuel of happy thoughts and focused energy.
There are plenty of things in life that aren't so pleasant, but demand our attention. Most people want to look and feel good, but they don't want to put in the effort to do so. Then something horrible happens, like the onset of Diabetes which is a serious disease easily controlled by healthy lifestyle changes. Your life depends on these changes.
Motivation in a negative exists and should be recognized. Your reasons for losing weight may be that you would like to look good in your skinny jeans, but if you have arthritis in your knees or chronic low back pain (which are all negative obviously), then surely your motives for losing weight should primarily be to eliminate these issues first. Maybe you want to add another 10+ years to your life, or be around to watch your grandkids graduate from college or beyond, perhaps you just want to manage stress, or lower your cholesterol to rid yourself of medication.
As long as negativity does not drag you down and adversely affect your progress, it is important that it drives you to change and progress. Think of something negative in your life that is an obstacle to positive change right now, how can you turn it around and allow it to help you reach new levels in your life today?