Frustrated baby while breastfeeding
Believe it or not, breastfeeding is just as hard for a baby as it is for the mom. They have to figure out the right way to hold their mouths, how hard to suck, and when to to swallow without choking all while their confused mama is shifting positions and trying to figure out her part. Not to mention those little guys get tired pretty quick and if there isn't enough milk coming out they can get pretty mad!
Older babies can get mad because they want to go play or they simply aren't hungry at that time. They could also be overly hungry and the milk just isn't coming fast enough for them. Teething can make nursing painful too, so that can upset them.
The best thing to do if you or your baby is getting mad while nursing is to take a break. The baby won't starve if you wait 15 minutes for everyone to calm down. I had to hand Avery over to my husband a few times just to get a breath before I tried again. It's also important to realize that your baby will nurse when they need to. don't force it.
I don't ever remember actually stopping nursing for a break, unless it was for biting. I found that their nursing, tugging, etc. helped my milk letdown. Though I have heard that the crying of a baby can help or trigger a mom's milk to letdown. That didn't happen very often for me for some reason.