Why do we as humans question why someone decides to alter their appearance physically?

Some do and some don't. I think one of the questions is why are we such a visual and image oriented society that people feel the need to do that.

Not all human societies were quite so absorbed in how they looked rather than what they did, what they learned, how they lived their lives.

For example, why exactly is Paris Hilton a celebrity?

In other societies, you would have needed more than a look and money, you would need an extraordinary skill to become famous.

People who question others who alter their appearance are thinking:

"why do they think all that is so important?"

For myself, I am ok with people doing what makes them happy, and I try to look professional myself, but I probably would draw the line on anything drastic, like surgery.

Probably because we are creatures of consistency. We tend to associate someone's looks with their personality and with what we know about them. Most people can handle minor changes, like changing hair color or style of dress. However, when the change is radical (multiple facial piercings, tattoos, dying the hair an unnatural color or adopting an unnatural hair stryle, etc.), this messes with our viewpoint of that person. It is no longer easy to assimilate their "new look" into our existing image of them.

Most of us can't financially afford to do it. Most people can't believe someone is so vain to have their appearance changed.