Life is a highway...
Yes, in many ways grief is a selfish expression of fear through ignorance.
We are not usually concerned for the current well being or not of the one who has lost their body, but of how we are now to cope without them.
Also, as you said it reminds us of our own physical mortality here and that we must also face "the great unknown" one day. This is especially true looking at the Math once you reach the "hump" age 35-40 where your likely days left are less than those already spent. This combined with the fact that you will have known more people that have died, makes death seems more real.