We get a lot of questions concerning sugar-substitutes such as stevia (marketed as Truvia), Splenda, Agave Nectar and the like.
Paraphrasing from a study by Purdue University: Why would a sugar substitute backfire? Sweet foods provide a "salient orosensory stimulus" that strongly predicts someone is about to take in a lot of calories. Ingestive and digestive reflexes gear up for that intake...but when false sweetness isn't followed by lots of calories (e.g. diet soda), the system gets confused. Thus, people may eat more or expend less energy thay they otherwise would.
In summary, artificial sweeteners can trigger the SAME insulin response as real sugar. The body tastes something sweet and doesn't know the difference. Fresh fruit should always be your first choice to satisfy that sweet tooth. Raw honey from a LOCAL Apiarist (beekeeper) is ok, but use sparingly.
Here is a great article concerning Agave Nectar, a popular sweetener used in some paleo-type recipes. Substitute fresh or dried fruit or honey when necessary.