why did the ancient mariner select that purticular guest to narrate his story ? why did he feel the guest would listen to him

His choice was random;

IT is an ancient Mariner,

And he stoppeth one of three.

"By thy long grey beard and glittering eye,

Now wherefore stopp'st thou me?

and his choice is mesmerized;

He holds him with his glittering eye --

The Wedding-Guest stood still,

And listens like a three years child:

The Mariner hath his will.

The Wedding-Guest sat on a stone:

He cannot chuse but hear;

And thus spake on that ancient man,

The bright-eyed Mariner