When the doctors tell you to rotate your injection sites DO, because if you don't lots of bad things can happen, trust me, I know.
From experience you can get very comfortable with a certain area, either for easy of access or it's less painful. It gets less painful because you can kill of the nerve endings in that site, so you don't always realize whats happening because the feeling isn't there anymore.
There's always the major risk of building up the insulin too. When you inject the insulin it absorbs into your body, if you inject into the same area each time (or the same few) then over time (and surprisingly it can be quick) the insulin builds up as it waits for the last lot to absorb, that's where the lumps come from. So you think your inject 10 units of insulin but in reality you might only be getting half of that because of the build up. So then you have to start injecting more and more to combat that which in turn makes the build up bigger and bigger, it's a real catch 22. I did this with my stomach when I was young. I disfigured my stomach horrible because of it, its taken nearly 20 years (and that's not exaggerating) to get back to a more normal shape, it's the one area of my body that I am most weary of. It also started to build up on my thighs to, but never as bad as my stomach.
Because of the build up and then because of the huge doses of insulin I had to inject, I became insulin resistant (which isn't what you need for an insulin dependent Diabetic lol)
But it isn't always injecting in the same sites that can build up insulin, it can also be the size of the needle your using.
Again when I was quite young I found the normal sized needles painful (I was stick thin, no meat on my bones at all) so my mum got specially ordered extra short needles, which was great for me because they didn't hurt as much, but the insulin wasn't going in deep enough so was taking longer to absorb. That also doesn't help with the lumps. So not only do you get the umps but you can also get bruising too and damage nerve endings.
Injecting isn't just dialing up how much insulin you need and sticking it into your body. It's a delicate balance that most forget about, but it can help so much for the future not only for your control but beings happy in yourself too.