The Italian teen, signed from Sassuolo, has broken into the BVB first team this season, made his Champions League debut and also has scored a first senior goal.
He told Cronache di Spogliatoio: "I spent eight years with the black-and-greens, a great journey. When they told me Borussia wanted me, I had practically already made up my mind, and I'm delighted with this choice.
"I certainly didn't expect things to grow so quickly, and I also thank the club for their belief in me."
On making his UCL debut against Atalanta, Reggiani also said: "It was special and it wasn't an easy experience because I was quite nervous, but my teammates really supported me throughout the game.
"Before the match, I spoke to (Honest) Ahanor, we spoke, and he asked me if I would play: when I said yes, he couldn't believe it, he was amazed and didn't expect it."
On his first goal against Augsburg, he added: "After the goal against Augsburg, (Nico) Schlotterbeck took me aside and pushed me against the Yellow Wall to get all the applause.
"It was a thrilling feeling because Dortmund's stadium is the most beautiful stadium in the world."