Well, I'm not an herbalist so don't quote me, but from what I have read, the reason you need to be sure meds don't mix with herbs is because you can get too much of the chemical from either the herb or the medicine building up (especially with SSRIs) if you take both at the same time, which can be very dangerous to your health. Or the herb you are taking can lessen the effect of other meds like heart medications, birth control or whatever you are taking. So, even though it's considered natural, it is a natural chemical and needs to be treated with precautions just like any other chemical you put into your body. So, make sure you ask your doctor and / or your herbalist to make sure there won't be any problems.
I'm not really sure why doctors don't suggest herbs rather than meds to tell you the truth. I know that some doctors suggest one or the other depending on the patient - my doctor does anyway. I do know that herbs are not regulated by the FDA the way meds are and that in itself may be one reason doctors don't suggest them - there's no FDA proof of herbal safety or the effects whether beneficial or not. Personally, I trust herbs more than I do meds though. Unless it's something life threatening, I go to herbs first.
One thing I forgot to mention before is that you also need to consider allergies. After all, herbs are plants.
A safe rule is to always do your research before you take anything - whether it's meds or herbs. :)