What is there other than this ultimate reunion of souls?
There is what we see, feel, hear, touch.
Why would life be worth living without eternity?
It is worth living for what we see, hear, feel, touch and otherwise experience.
Are the feelings of total partiality, and love all part of humanity's innate abilities to become one with eachother?
They are partly innate abilities, and partly acquired (learned).
And why are only humans like this? Why do the polar bears not cry?
I'm not convinced it is only humans. I have heard of cases where dogs have mourned.
Our feelings are partly a result of learning, of being part of a cooperative social group. We are an intensely social species.
Why are we special, and who/what made us like this?
We have appointed ourselves special. You could say that it is a matter of ego.