/sci/ - Science & Math

idk if this is the best place to ask this but google searching tends to either give me scientific papers that aren't related, or "webmd" type how-to articles.

i'm diabetic and i give myself regular subcutaneous injections in the abdomen. i use a small 31guage needle. most of the time, there's a mild amount of pain. like if you were to take a knife tip and touch your finger, pressing gently.
however, sometimes, there's quite a bit of pain and resistance from the needle going in. as if the skin catches the needle without penetrating, and it takes a bit of force to break the skin. often i'll move somewhere else and give it another try.
other times, it's totally painless to the extent that if i wasn't paying attention, i wouldn't know i'd been stuck with a needle.
my question is, why is there sometimes pain, and sometimes not? is it technique? my intuition would tell me that it's just chance based on how dense nerve endings are in the region. if i get a low-density region, there is no pain detected.
however, why are mosquito bites painless? i know there is anesthetic action in the saliva, but the penetration itself is not noticeable.
i feel like the same explanation is underlying both of these questions.