The prepositional phrases, "To many people" and "For many people," are grammatically correct. The clause that follows, "dogs are the best friends" is grammatically correct. The sentences are grammatically correct.

Is the difference in meaning between “To many people” and “For many people,” dependent on the clause that follows? A different example makes it clearer. "For many people, water is wet [when experiencing water]." "To many people, water is wet [in the view of]." When this distinction is applied to the original sentences, “For many people, dogs are best friends.” means in the experience of many people. “To many people, dogs are best friends.” means in the view of many people.

It depends on whether you want to emphasize experience or view.

The original sentiment, "A dog is a man's best friend." emphasizes the superiority of a dog over a person as a best friend. Many would agree.

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