Different reasons motivate different people: sometimes it's pleasure ("I want to fit into my skinny jeans") and sometimes it's fear ("If I don't lose this weight, I may become diabetic"). You have to decide what's most motivating for you personally. I also recommend being positive when you make this list, phrasing it something like, "Once I lose fifteen pounds, I will fit into that size 8 dress" versus "I can only shop in the fat store." In my experience, people commonly say, "I'm doing it for my health," which is actually very abstract, so instead, I ask them to detail how their weight creates a negative impact on their lives.
For people who are not exceptionally overweight, you may need to motivate yourself with other things, like fitting into your skinny jeans. Some people depend on external motivators, like a wedding. But that only lasts until the date comes and goes -- it's not a permanent motivator. So I suggest looking at the bigger picture and finding an internal motivator, like being able to walk longer distances, climbing stairs without huffing and puffing, and losing that extra fat around your gut that is increasing the likelihood of disease.
Once you have identified your motivators and created your list, keep it with you, in your wallet or purse. You can either read it as a mantra every day or carry it with you for moments when you feel weak and need a positive reminder of why you're doing what you're doing. When you feel down or uninspired, just look at the list!
Find out more about this book:Eating Free: The Carb-Friendly Way to Lose Inches, Embrace Your Hunger, and Keep Weight Off for Good