Why do we need to love one another? If there is no afterlife, why must we leave live? If we all are just going to die and be forgotten, that makes life rather pointless.
(I edited this question, to remove needless shock statements)
Asking why we "need to love one another" is probably the wrong question. The short answer is, we don't. We do, however, but why?
We're a social species. Part of our evolutionary history was banding together, defending each other, helping each other out, etc. That's part of our survival (at least, within tribes). You can see it in chipmunks too. Usually, they'll violently squabble over territory, until it's mating season. Then, they'll pair up for awhile be a couple, and then part ways. When the offspring are born, they'll be tolerated and looked after, until they venture out. It's part of their instinct.
There's not a lot of mystery to it... but I'd personally argue that humans aren't much better at love than chipmunks.
Why must we live? Again, it's looking at it backwards. We're compelled to live because the species that didn't have a survival instinct went extinct. Beyond that, it's up to the individual as to why they want to live, above and beyond their own inhibitions.