Can one sacrifice any permanent at any time for any reason, or is "permission" to sacrifice only granted by other cards?
You can't spontaneously sacrifice a permanent. Even if you have a permanent with an ability of the form "whenever a creature dies" or "whenever a creature is sacrificed", you need to find some means of sacrificing it.
There's no rule forbidding it; there's simply no rule allowing it. CR 116 itemizes what actions you can perform. When you have priority, you can do the following:
- 116.1a Cast a spell.
- 116.1b Activate an activated ability.
- 116.1c Perform certain Special Actions.
- 115.2a Play a land.
- 115.2b Turn a face-down creature face up.
- 115.2c Take an action an effect previously permitted you to take at a later time (usually to end a continuous effect or to stop a delayed triggered ability from triggering).
- 115.2d Take an action a static ability allows you to take to ignore the effect of that static ability for a duration.
- 115.2e Exile a card with Suspend.
- 115.2f Role the Planar Die.
- 115.2g Turn a face-down conspiracy card face up.
- 116.1d Activate a mana ability.
As you can see, sacrificing a Permanent is not an option (unless it's part of one of the above actions (e.g. The cost to activate an ability)).
- There are other actions you can take (e.g. Drawing a card in your Draw Step), but they aren't actions you chose to do, so I won't bother listing them. They are itemized by CR 116.2.
- Conditions may apply.
You can only sacrifice a permanent when instructed to do so by some ability. Even if you had another ability such as "when a creature is sacrificed...", you would need something else to let you sacrifice a creature.
I would say that if there isn't anything saying you can, then you probably just can't do it. If you could sacrifice a permanent any time you wanted, you would make all enchantments that hurt you directly, worthless. I did that in a game. Grant it, however, it was a very nice creature, and it sucked that I had to sacrifice it, but I did that rather than taking the 2 life of damage each upkeep. I'd say it could make a nice house rule though, because it's not broken. You could put nasty enchantments on the best creatures your opponent controls, and they'd sacrifice it. Then again, with any black deck, you could more than likely bring it back one way or another. It would be a matter of preference at that point.