Ponder this hypothetical situation.
A nation has an army and security apparatus of a finite size. A nation also has certain internal security issues. Suppose a potentially hostile force stations troops near one of the nation's borders. The nation's natural reaction would be to increase it's own presence near that border. Where do the additional forces come from? Other, quieter borders or from internal security duty. Now what if the majority of the nation's borders have potentially hostile forces stationed near them. There is an even greater need for personnel to increase the military presence on those borders. At the same time, there are suddenly not as many quiet borders to pull personnel from, which leaves only those on internal security duty to draw from.
Not to say that this is representative of Iran's situation, but it's an illustration of a certain type of situation that has plauged some nations in the past.