'98 ML320 brakes questions

[Homer]

I recently replaced the front brake pads at around 80,000 miles. I noticed the passenger side pads were a little more worm than the driver side. As pointed out the job isn't hard, I found that a 6 inch c clamp is a perfect size to re-compress the piston by the way. At any rate everything was fine for a few miles then the passenger side front brake started to drag. I re-compressed the piston and this was good for 60 miles when it again started dragging.

My uninformed questions are,
1 Is it the caliper that is bad and not something more involved "up the line"?
2 Why do the parts houses say that both calipers should be replaced at the same time?
3 This same type problem, ie dragging brakes, occured while the beast was under warrenty so I didn't pay much attention to what was done, and I can't find the invoice. Is there something wrong with the '98 brakes?

Any comments would be appreciated.