You and Crona have had the very bad luck that you were cared for together. That is, they both had the same childhood torture of Medusa, with the slight difference that (at least) you didn't have a demon inside.

The fact that Crona has developed emotional dependence on you was not unexpected, since you are literally the only person outside of family ties who was with them since childhood. Even when they managed to be in Death Weapon Meister Academy. "I.. can't do anything without you..."