Many creative writing classes and manuals will stress the importance of the first sentence, that it must grab the reader’s attention. Even Hemingway espoused this fallacy. But the first sentence is never enough.
Yes, it must grab the reader’s attention, If it doesn’t the reader will go elsewhere. There are plenty of options — but if the second sentence is flaccid, all will be lost. The second sentence fulfills the promise of the first.
But it is the third sentence that seals the deal. The third sentence assures the reader that the writer is authentic, that they are worth listening to, that they have something to say and have the command of language to say it with flair and authority. They can be trusted.
After that the writer will be ‘in full swing’. The reader will be committed; will go along for the ride.