Very generally I think spying happens because nations and non-state groups have interests that are often not aligned with each other, and nations protect their own interests by trying to understand the interests of other nations. Since very few nations are totally up-front about what their interest, goals and behind the scenes actions are, intelligence gathering becomes necessary.
In a perfect world, this wouldn't be necessary. Not only would countries be totally open with each other, but everyone would work together to further the best interests of all. Since we don't live in that kind of world, everyone gathers intelligence on everyone else.
I think it's worth mentioning that people have made the (reasonable, IMO) claim that spying in general actually makes the world a safer place since it removes a great deal of uncertainty in international relations. If countries had to deal with the rest of the world as a black box with uncertain motivations and potentially hidden actions, I think it's reasonable to suggest that mutual suspicion would significantly impede international cooperation.
In a perfect world, this wouldn't be necessary. Not only would countries be totally open with each other, but everyone would work together to further the best interests of all. Since we don't live in that kind of world, everyone gathers intelligence on everyone else.
I think it's worth mentioning that people have made the (reasonable, IMO) claim that spying in general actually makes the world a safer place since it removes a great deal of uncertainty in international relations. If countries had to deal with the rest of the world as a black box with uncertain motivations and potentially hidden actions, I think it's reasonable to suggest that mutual suspicion would significantly impede international cooperation.