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Ok, heres how it works....
Scavengers.....
Millions of years ago........
There are wee scavenger fish which, because of their habitat perhaps don't provide a big prey opportunity for sharks, and which don't play any other part in the shark's life.
Along comes a shark.
It has fed recently and isn't particularly interested in tiny fish which would need more energy to catch than they'd provide as a meal, and which can disappear into some dangerously jaggy coral before you can blink.
The shark is plagued with skin parasites and rotting detritus in its mouth. The shark doesn't know this. It only knows an itch or whatever signals its nervous system generates.
The sites of infection leak pus or blood releasing chemicals in the water which say "food" to the scavenger fish, some of which go and have a wee pick at the stuff.
The shark may well kill and eat a few of them. Some will survive for a variety of reasons, perhaps down to their particular colours or scent making them difficult for the shark to detect and catch, or by ingesting enough of the shark's discarded skin particles and associated parasites they avoid recognition as a potential food item by emitting similar chemicals to the shark itself - in effect they smell like a shark.
Some of the ones that survive pass on the traits that enable them to do this to their offspring.
Bear in mind that not all of the scavenger fish will go for this. A population will continue feeding the way they did before the sharks appeared, but the ones who adapt will continue to do this down the generations until they have evolved enough to be a completely different species. Darwin's famous Galapagos finches are a prime example of this kind of evolution.
You fill whatever niche give your genes the best chance to survive, and if that means exploiting a brand new food opportunity where others can't, you're hitting the ground running in evolutionary terms.
To go back to the sharks themselves, animals which are full of parasites are going to be at a disadvantage in the feeding and breeding stakes.
The ones which develop a trait which allows scavenger fish to work on them are the ones which ultimately will survive and the habit will become normal behaviour for that kind of shark and that species of scavenger.
As the symbiosis develops, competition between individuals will encourage the development of more traits to aid the efficiency of both partners.
For example some modern sharks regularly visit reefs which are populated by particular varieties of cleaner fish, perhaps with specialist mouthparts, or colours or scents which provide a stronger stimulus to the shark's recognition of the service being provided.
Notice I am in no way suggesting that this is a conscious decision on the part of the shark. Such behaviour is survival-driven.
It's a mechanism.
No psychic messages required.
None.
Zilch.
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