Human beings are curious by nature, we love to know and we do by asking questions for reference (sometimes for better gossip) we sometimes cross the lines and because we are so used to this, we assume it is normal.
There should be boundaries/ rules of etiquette that should essentially guide us to be better (and appropriate) people who respect others, just because that is the way it is meant to be.
It is rude to blurt out to a person for instance 'how much do you make'? This is not only intruding in another's private business, it is just rude except if they offer to tell you, you are otherwise supposed not to ask. It is their business not yours!
How much our friend's sofa, pairs of shoes etc cost is not our business, our business is to admire it if we like it. If we admire it (and possibly want to get same) and they offer to tell us how much it costs then fine otherwise it is none of yours!
Asking how much do you weigh is like saying 'you are fat or skinny!' why don't you let them bring it up in conversations if they want (or not) asking them is going out of line even if its from your bestfriend.
It is rude to ask people how old they are! Have you heard the saying, you don't ask a lady how old she is!
5. Sex Life
It is just rude when you ask people what they do behind closed doors, its offensive, what they do is their business and except you are one part of the relationship, shut the front door!
You should never ask someone how far along they are when pregnant or when they are due. When the time comes you will definitely know.
Don't ask people why they are not married yet it is like asking 'you know you are sick for being single right'. Don't ask that.
By all means respect people's privacy, there is a reason it is so called that way we are on our way to being gentlemen and ladies.