Act one Scene one, Bassanio's Childhood

What is the purpose behind Bassanio mentioning his Childhood memories to Antonio in Act one Scene One?

" In my school-days, when I had lost one shaft,
I shot his fellow of the self-same flight
The self-same way with more advised watch,
To find the other forth, and by adventuring both
I oft found both: I urge this childhood proof,
Because what follows is pure innocence.
I owe you much, and, like a wilful youth,
That which I owe is lost; but if you please
To shoot another arrow that self way
Which you did shoot the first, I do not doubt,
As I will watch the aim, or to find both
Or bring your latter hazard back again
And thankfully rest debtor for the first. "