unrelated- why we feel grief

It seems at first glance that grief be a negative evolutionary trait. It doesn't seem to effect those creatures that we consider "lower" on the evolutionary scale. So from this I would typically assume that thanks to natural selection and survival of the fittest- grief would have disappeared as an emotion somewhere down the line. It leaves us weak for a short while. I would imagine this temporary weakness would make an ancient family of primates vulnerable to attack from enemies. Any competing tribe would have to do is kill off the weakest family member, and attack them while they're busy grieving. So what is it good for anyways? The reptiles don't seem to have it, the other mice never seem to miss a beat after one of their comrades has been snagged by a trap. I did some searching and found one article that summed it all up here

So basically it has two theories, the first of which I feel is pretty solid, the second but I'm not as keen on.

First idea is that if you're aware that a member of your genetic family goes away, they will be unable to send their genetic makeup (AKA your genetic makeup) down the line. So put simply creatures who don't feel grief would be less inclined toward saving their family members from certain doom. They may even be more likely to see their kin as a source of food. Creatures who work together live longer, and therefore father more children.

The second idea that I wasn't as keen on but I will mention it is the idea that creatures who display grief are considered by possible mates to be more loyal. It's a long shot in my mind but I'm sure it didn't hurt griefs' chances of surviving natural selection.