What is the US FAA concerned about when it comes to psychiatric medications? I don't understand the FAA's policy for psychiatric medications (specifically anti-depressants). Obviously I'm glad there is oversight into who can obtain a pilot's license for the safety of the pilot, crew, passengers, and the people on the ground. And obviously people need to be in a stable condition before they eligible to fly. But some of these rules sound ridiculous to me.

First, they won't approve you to fly while taking a psychiatric medication, but they may approve you to fly if you decide to go off of it for 60 days.

What?

So basically if you have a chronic mental health condition, and you are taking a medication for it, and it's working well without side effects, it's better to go off the medication before you try flying? Wouldn't the disease be more apt to return if you were off the medication, than on it? That sounds like a very stupid risk for the FAA to take.

Are they worried about side-effects? Most of the side effects for modern SSRIs are very limited and/or transient and/or don't apply to flying (like constipation), so I know they can't be worried about those.

Are they just concerned about withdrawals if you miss a dose? That seems pretty unlikely to happen, and probably less likely to cause trouble than to go off meds entirely.

Basically, I don't get it. It seems that if you are fully-treated for a condition without side effects that you should be eligible to fly, regardless of whatever drugs are put in your body. Honestly, I'd be more concerned if people were taking less medications for passing a test, than what they were on before.
The FAA is concerned that individuals taking anti-depressant medication will not have the mental focus required when juggling multiple tasks in real time in the cockpit.

anti-depressant medications are psychotropic -- They are specifically intended to alter the takers' mental state.  The assumption is that any artificial modification of mental state will negatively impact aeronautical decision making. 

The SSRI special issuance protocol requires, among other things, the applicant to demonstrate the ability to quickly and accurately perform multiple tasks simultaneously.  The idea is to make sure that you aren't being slowed down by the medications to the point where you can't make the right decisions rapidly enough to ensure safety of flight.