Goroutine crashes silently

Question

I'm having some issues with a script of mine that dies silently once I try to make an existing function a goroutine. It seems if there is only a go routine in the main function then it doesn't run. Why is that?

After some fooling around I tried to reproduce the issue.

Heres a simple case from tour of

https://play.golang.org/p/78MQ90pp5L

package main

import (
    "fmt"
    "time"
)

func say(s string) {
    for i := 0; i < 5; i++ {
        time.Sleep(100 * time.Millisecond)
        fmt.Println(s)
    }
}

func main() {
    go say("world")
    say("hello")
}

If I comment say("hello") the program exits prematurely. Can someone explain why?