Human beings are plenty loving

[Infinite]

It's a complex question.

Ra said that understading is not from third density because here there's the veil of forgetting (which I suposse you know the concept). In 4D there's no this veil and is the first density with self-awareness without the forgetting of spiritual reality. I believe this is the meaning of "understanding".

About love, you probably (and me too) don't know the real love. This is the unconditional love which only can be experiencied with the kundalini seated on the heart chakra. That's the requisite to harvest. In hindu sources is said that when kundalini reaches the heart chakra occurs the first spirtual awakening. Because the truth that we are one become obvious to the entity. Many others things occurs with the entity. So, I really believe this state is the begining of the "understanding".

For this reason, the Confederation and the vast majority of spirtual sources talks about love: because you need reach the vibrational characteristic of the next density to become able to the quantum leap to it. The fourth density is about love and so, we need reach the love to experience it.