Unless the mechanic believes the working caliper is showing signs of seizing in the very near future, NO you do not need to replace both calipers. Brake rotors and pads should be replaced as sets, but normally a single caliper can be replaced or rebuilt.

Not knowing the estimate or what vehicle you drive, I can’t say if the estimate was unreasonable.

I am also being quoted a lot of money, much more than 400, for total job, both sides.

Does it seem reasonable to pay a lot more than $$400 US, for this? thanks.

Also, cause, I am being told it “just happens sometimes.? Is that plausible?