Can I install ceramic tile on top of existing ceramic tile on bathroom floor?

I heard mixed reviews on this idea. Please tell me how to install tile over tile. For the people who say it's a bad idea, please explain why using ceramic tile as a subfloor is inferior to a concrete or wood subfloor? There has to be a legitimate explanation other than "it's not the right way"
Update: I don't have to worry about leveling, because the floor is already level from when the first tiles were installed.

Plus, there is an adhesive for EVERYTHING. You can't truly say that the new tile won't stick to the old tile. That's like saying the tile surface is ADHESIVE RESISTANT.
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