[Basic_Strategy]

Why would anyone wants to spy on it's allies.........................? they should spy on their enemies......................

Allies also won't be 100% open book with each other. 

Sometimes spying on allies isn't done with intent of malice, but to keep tabs on their internal stuff so in case anything potentially malicious comes up internally to threaten the internal stability of one's allies or the spying country's interests/relations with allies, the spying country won't get caught unaware and can make contingency plans. 

Actually there's an official term for this: it's called backchannelling. Stuff that never gets openly acknowledged. Very undertable friendships and connections that will maintain plausible deniability at all costs between high-ranking internal officials on both sides, but they will talk to each other and swap intelligence on their own internal affairs and issues.