If you are using the same site over and over the skin can become thickened and fiberous and the needles don't go in as smoothly. Also, the fat pad decreases in sites used frequently, which makes it more likely to get into the muscle which is sensitive. Rotating sites can help prevent these area from becoming painful. If you are using lantus and increased the dose recently, larger doses of lantus can sting. Also, if you opened a new box of needles recently, or are using a new brand, that can make a huge difference. Max used to get very irritated from the generic brands of longer needles, so I got the expensive short, ultrafine BD needles and that helped immensely. They do cost a small fortune, however, but it was worth it for us.