Variable value not set?

DECLARE @validation_message NVARCHAR(200)

BEGIN 
	SET @validation_message = @validation_message + 'test was here'
END

SELECT @validation_message

What silly mistake am I making?  Why is my variable value null?

However, If I change the code to this, I get the expected result

DECLARE @validation_message NVARCHAR(200)
SET @validation_message = ''
BEGIN 
	SET @validation_message = @validation_message + 'test was here'
END

SELECT @validation_message