Why is planning a wedding so stressful?

It's due to the spam involved.

Not spam, collecting valuable information from the intelligence of A2K's past Brides/Grooms, for a wonderful, helpful, new website, or Forum, or shirt that's spam

I did not get stressed over my wedding by the way in planning, setting up, putting a dress on, to drinking a glass of champs first.

Nup.

Think of it is this way -- from the viewpoint of the bride and sometimes grooms also get involved and even other family members -- any way you are probably planning the largest party/gathering that you ever had before so just the huge size of it, for many brides they consider this the most important day of their lives (at least so far) so they strive for perfection on the day; the potentially huge cost depending on size and other things you make it.

Family can increase stress if they give too high expectations as well.

Best thing - keep things in perspective. Realize what is really important about the day and the other things are just bonus. I think many brides and often grooms put the stress on themselves if you focus on what is really important the other things tend to fall into place and you don't worry about it being perfect. Who cares if the flowers get messed up for example, the wedding will still take place and the important thing is the actual wedding ceremony and the commitment to the couple - the other things although fun and great and so forth is not any where as near important.