Potential reasons not to rely on the US: - They become your military adversary. - Some autocrat-minded person gets elected as President of the US and refuses

Potential reasons not to rely on the US: - They become your military adversary. - Some autocrat-minded person gets elected as President of the US and refuses to provide the EU money, services, and weapons until the EU complies with some demand against it's own interests. - Perhaps in combo with the second item, the US could lead the EU on, to further it's interests, and let the EU down at the worst possible times.