Pensieve > Unforgivable

What makes an Unforgivable curse so unforgivable? You can cause anyone pain with many of the spells mentioned in the books. I doubt that anyone would enjoy their hair being set aflame or their clothing being shrunk so much that breathing was restricted. What about being levitated off a cliff or a huge weight being leivated over you and then dropped? The possibilities are, sadly, endless. So why are the Unforgiables so special?

At their core, regular spells are created with no maligned intent. Fire spells to light candles and fires, to help in cooking. Shrinking and growing spells to reuse clothing and make other things a better size for use or transport. Levitation to make transporting objects even easier. Most people don't think to use these spells against one another. (Unless you happen to be a pranskter, but even then you're not likely to truly wish death upon your mark.)

Unforgivables, however, were not so innocently created, nor can they be so casually used. They exert control over another being, against its will. Perhaps that is what is so unforgivable; taking away control from another being.

The first, Imperius, simply imposes one's will on to another. It takes away a person's ability to choose, an ability that defines and shapes us as human beings. Using this curse essentially puts the victim's life into the caster's hands, as they could be made to do anything, from something as mundane as skipping down the lane to more dangerous pursuits, like jumping off a rooftop.

The second, Crutatius, is torture in its purest form. Every nerve-ending going off at once cannot be pleasant, and it seems unlikely that there is any other use for this curse other than the inducement of fear. If used enough, it can even render the victim insane, destroying who they are as a person.

The final, the Killing Curse, is the ultimate control of another. There is no chance for recovery; life is slammed to a complete stop by instant death.

Aside from the evil nature of their results and use, the main reason that they are unforgivable is that one has to mean it when casting one. There has to be intent behind the spell in order for it to work correctly. And it is the desire to harm another that is truly unforgivable.