The England defender has established himself as a first-choice at Bayer in his first season in Germany.
"It was the management and the way they approached me because they had a very clear plan of what they’d been watching and everything they saw from me," he told Sky Sports News about his move.
"They had so much detail in terms of what they’d seen of me, the potential they thought I could live up to and it matched up to what I was thinking as well. Seeing as we were on the same page in that sense, it was a no-brainer.
"To be honest, I wouldn’t say it was the hardest decision because I just wanted to play. I felt like I could play at the top level, the Bundesliga’s a top league and being able to play in the Champions League and play top games."