Why do positive and negative P/T counters remove each other? You have a 2/2 creature with a +1/+1 counter on it. I play Spitting Dilophosaurus, targeting your creature, put a -1/-1 counter on it. Spitting says “Creatures your opponents control with -1/-1 counters on them can’t block.” Can your creature block? Yes, according to 122.3. Why don’t counters cancel each other out without being removed? Shouldn’t you have the opportunity (e.g., Ferropede) to remove my -1/-1 and turn that +1/+1 back on?

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Counters cancel each other out because it simplifies tracking what’s going on.