Bursting a cold sore blister is one of the worst things that you can do. Besides the fact that it will hurt, you will be giving the virus fluid a chance to spread to other parts of the mouth and lips.
You could also infect the cheeks, nose, eyes, fingers, and brain (less likely). And the last thing that you want to do is infect your family, friends, and work colleagues. It can and does happen when you start squeezing cold sores to remove the fluid.