Because people believe they do.
In each day of your life, a few somewhat good things and a few somewhat bad things happen. And even a real good or real bad thing may occur. But you typically don't even notice.
If you are "cursed", and believe it, then you'll be worried, and keeping a look out for anything bad that happens. And you'll notice those ordinary bad things, and figure they were curse caused.
On the other hand, if someone had gave you a "lucky charm", you would probably look for the good things - and notice those.
No, they do not exist.
yes
Not in the sense of bringing misfortune to someone else.
Curses don't exist, so I wouldn't worry too much.
There are none.
Count Your Curses was created in 2011.
Satanic Curses was created in 2007.
Water Curses was created in 2007.
Curses and Countercurses is in Chapter 12, Moment 2 (Christmas at Hogwarts), lying on the bench.
Generational curses are those curses that do not end when a person dies. They are curses that follow the children, grandchildren, and great grandchildren of those who were cursed. They are passed on from one generation to the next.
There is a lot of information about curses found on the internet. Wikipedia has tons of information on curses as well as sites like How Stuff Works. Of course there is no scientific evidence for curses.
The collective noun for curses is often referred to as a "litany" of curses. This term captures the idea of a series or list of curses being expressed together. Other terms like "heap" or "string" of curses can also be used informally, but "litany" is the most commonly accepted.