Both depression and effects of meds are an individual thing; not all antidepressants work for all people, etc. If a non-depressed person took an antidepressant drug, it wouldn't "work" and there would seem to be more side effects. It's like taking any other drug/med. If you take an Advil/Tylenol when you don't have a headache, what happens?

Like Special K implies, the opposite of depressed isn't "happy." It's more like "normal" for you as an individual. You feel less depressed, which we sometimes think of as "sad" but that doesn't mean you feel more happy. There's just a bit more "interest" and energy. If you already have enough of the brain chemicals you need, messing with them can make you nervous or sleepy, etc., like some of the side effects listed usually. That's why those side effects happen; not everyone prescribed a particular med is going to be able to use that one or have it work. They can't "measure" and look at depression like they can other body chemical or hormonal imbalances (they can tell when you're diabetic or have an infection and need antibiotics, for example).