Kapitel 1659: The Next Step After The Era Of Conquest.

He is not happy because he gained a single realm heart. He has a lot of realm heart fragments. He is very rich. What he is happy about is gaining a complete realm heart. His wealth can't buy that.

Realm hearts break up whenever the realm is destroyed, so only fragments can be acquired from the destruction of the realm heart. Things like the spark of consciousness won't be preserved and will be destroyed.

This spark of consciousness is very important to him, as it is one of the requirements for him to be able to awaken the powerful psychic engram sleeping within him. This is probably the most important reason why he had to become a realm lord. That and the power of the world he would gain as a realm lord who became a world god. 

It is only in the situation of the era of conquest that one can gain a complete realm heart. There's no other way to acquire a complete realm heart after the destruction of the realm.