Why do people reschedule TV shows?

I don't have that many TV shows I watch on a regular basis, so why is it that two of my favorites, the OC and Grey's Anatomy are being scheduled for the same time next season? I'd like to drop Grey's, just for the principle that they changed their schedule when I am used to watching it on Sunday night. However, my favorite charachter on the OC died, so their isn't much point of watching it anymore. Why is my life so complicated??!!