Klopp and Hummels won titles together at BVB and the Reds manager says: "We have always kept in touch. He is a natural leader.
"He is the best German central defender. I was happy about his return to Dortmund. It just fits.
"For me, this is a very logical story, that in such a long career as Mats will have had at the end, he stays away for three years and goes to a club where obviously you can still win titles - and he comes back to his original place."
Asked if he tried to sign Hummels, Klopp added: "No, because we already have four world-class centre-backs."