Remaining friends with someone after a relationship ends is difficult, and it's hard to not slip back into old habits, mannerisms, and step beyond the bounds of friendship once more.
Not to mention that most of the people you will date will feel uncomfortable with you keeping an ex around as a friend. To the point of feeling threatened.
"Well, then," the Cat went on, "you see a dog growls when it's angry, and wags its tail when it's pleased. Now I growl when I'm pleased, and wag my tail when I'm angry. Therefore I'm mad."