Pfft. It's directly stated in Master of Mankind. No need to wait for G-man to walk again. Malcador says it, straight up.
We learn it from The Emperor himself.We learn from Roboute that Emperor always considered primarchs more as tools.
We learn it from Dorn, who tried to earn his Father's love, who ultimately had to accept that he would never get it.
My guess is two-fold:So if we take that story at face value... why did Emperor held himself back?
1. The Emperor's heart grew three sizes that day, or
2. Horus really was his favourite.