Aggressive unlatch

My baby is about 8 weeks old, and I've been struggling with him unlatching when he's done eating. He pulls off on his own when he's done and maybe two out of every three times he tries to take my nipple with him. I don't really like to take him off myself unless he's taking a long time to eat, as I have a hard time telling how much he's had. But how do I prevent this behavior? It always surprises me and there don't seem to be any signs that he's about to do it​. Is this just another aspect of breast feeding to get used to?