What fallacy is it when someone says "this is true/it happened, therefore there are good reasons for it"?

For example: 

> 
> We drive on the left (or right, depending on country) side of the
>  road, therefore there must be a good reason for it.
> 
> 
> 

Assume, for the sake of argument, that both sides are an equally good choice.

The fallacy part is the claim that reasons exist when in reality they don't. The original event/fact is the result of an arbitrary decision. There were no "good reasons".