Well in atheism it is not just the passing of their physical form. It is the end to their existence. They are from the point of their death. Non-existent.
My observation is why would they have sentimentality or any other emotions. It serves no survival purpose. If someone has ceased to exists then they are no longer of any survival benefit to the group so the time spend investing emotional energy such as sentimentality detracts from their main goal which is survival.
People who spend weeks in mourning are not as productive as others. If the main motivator of human evolution is survival then such emotions would have never evolved. When as person dies is should be to others as if that person never existed at all. Emotional investment in others is only of use if they are alive to offer mutual support. Once they are dead then they are useless to you.
All Praise The Ancient Of Days
Given the day I'm having so far, I can find no words to describe this logic other than "Plain fucking stupid".
Emotion is not the exclusive domain of theists.
Missing someone - whether they be gone for a few days, or dead, is not the exclusive domain of theists.
Oh, and for the record, the emotions of mourning someone's loss, or missing them whether they be on holiday or dead
does in fact have survival value - emotional attachment to members of a group encourages cohesiveness of that group. The feeling of 'loss' when a member of a family group goes missing, either temporarily or permanently encourages survival by encouraging co-operation within the group, protection of each other, and encouraging the group to go looking for it's members who have been seperated from the group (encouraging a kind of a herd mentality through emotional attachement).
So not only is your reasoning moronic, and based in logical fallacies (strawman hypotheses). It's also flat out wrong.
Addendum:
Altruism.
That's the word I was looking for.
Emotional attachment, and the sense of loss associated with it serve a purpose with regards to survival by encouraging a herd like mentality (whether it be at the family, tribal, or recently national), and a degree of altruism and loyalty