Here's something that has always baffled me. Why is it bad to have open ports on your computer? Assuming you don't have a virus on your computer or some other program listening to a port that may actually do something why does it matter if a port is open? If some malicious person starts sending packets to a port, but nothing is there to receive the data and do something with it why does it matter? A computer doesn't just arbitrarly execute any data it receives. I understand flooding a computer with packets could cause it to crash because it just can't deal with the amount of data, but I'm only considering security issues that actually change the files on the computer.
Update
Thanks for the replies so far. I understand now, that a port being open actually means there is a program that can be exploited listening on that port. But why is it so hard to write software that can't be exploited? To do any real harm to a computer wouldn't a program have to allow one or more files to be uploaded, and then one of those files would need to be executed. It seems like that would be incredibly hard to allow accidentally.