By Matthew Cooper Jadon Sancho says his goal is “to be happy again” at Borussia Dortmund, after assisting Marco Reus in their 3-0 win over Darmstadt. Sancho rejoined Dortmund on loan from Manchester United two days ago, having been frozen out at Old Trafford after a public falling out with manager Erik ten Hag. As a result, Sancho has returned to Dortmund and he made an immediate impact after being introduced off the bench by manager Edin Terzic. In his first competitive appearance since last August, Sancho bagged an assist as he set up Reus for Dortmund’s second goal of the game. And the wing…