Shallow infatuation, eh? As opposed to what?
Their relationships are similar because of their function in the plot; the ladies die and provide impetus for the heroes to do what they do. Aeris and Rachel are pretty much exactly the same. There are also a plethora of reasons why Aeris is unrevivable; Phoenix Downs only work if the character is knocked unconscious. Notice if you will, on the Status screen, if a character has 0 HP they are listed as "KO" not "Dead." Phoenix Downs can heal anything from gunshots to severe medical trauma, but Aeris just outright died. If this were not true, Locke could have simply used a Phoenix Down on Rachel. For that matter, every dead character in Final Fantasy could be resurrected. As this is not the case, one is forced to go to outrageous lengths, such as Locke did, and even that didn't sustain her for more than a few minutes.
Compared to Sephiroth, the characters are also exceptionally weak at that point in time. Him stabbing Aeris was the equivalent of dropping a nuclear bomb to get rid of an anthill. If that doesn't kill you, nothing will.