Too much of anything is bad. A quart of table salt would kill you. A bucket of water force-fed into you could kill you.

Hemoglobin in blood contains a lot of iron; it's used to bind oxygen. Too much iron intake apparently can result in its overproduction, and too much is no good. Donating blood rids you of excess iron, while also benefiting other people.

I suppose you should first check if your levels of iron are indeed excessive.