What specific reaction are you referring to? Fire isn't the only natural source of heat. Why is evolutionary pressure from fire required for animals to evolve a sense of "something is too hot"? Why is it not sufficient for animals to evolve this due to e.g. exposure to sunlight, with the same mechanism being triggered through fire?
So the lobster feeling that they should ideally move to cooler water makes sense. Evolutionarily it wouldn’t surprise me if they don’t feel like they are in an emergency situation in the same way that a human would if they are standing in a fire.
Is there a "dangerous heat" environmental candidate for lobsters that we can speculate could be sensory-cross-triggered by boiling water? Underwater lava?
http://www.columbia.edu/~col8/lobsterarticle.pdf
Lobsters definitely feel pain at least as a stimulus responding to extreme heat.
Lobsters have nociceptors, special pain-receptors that are “sensitive to potentially damaging extremes of temperature, to mechanical forces, and to chemical substanceswhich are released when body tissues are damaged.”
Humans have similar receptors. Consider if you touch an extremely hot stove and remove your hand before you even realized what happened. That (temperature) pain stimulus is sent to your spinal cord (not your cortex) and then responds by moving your hand before you are consciously aware (through the cortex) of what is going on.
Lobsters do not have anything like the cortex which is involved in pain interpreation for humans (and also pain supression like in extremely traumatic events where a person doesn't feel any pain immediately after losing a limb).
The lack of a pain suppression mechanism is a little scary to think about in the context of being boiled alive, but this could potentially be because lobsters' experience of pain is so radically different from our own that they may not even have use of a pain supressor.
There are interesting anecdotes about people who received frontal lobotomies experiencing pain as a neutral experience that is neither bad or good. As in they feel pain from, but it is not perceived or interpreted as "bad" by them.
Also in general, lobsters are unusually sensitive to temperature among invertebrates, responding to temperature changes in lab studies as small as 1°, and their seasonal migratory patterns also seem to highly motivated by finding their preferred temperatures.
If you drop a lobster in a container of room temperature sea-water, the lobster just kind of sits at the bottom looking for the darkest most isolated space if there is one (normal behaviour) vs boiling sea-water you will notice lobsters trying to use their claws to not be fully submerged, push off the lid, etc... in the 35-45 seconds it takes for boiling water to kill them.
p.s. I hope this response is useful, I wrote it partially to let the mentioned article I just read sink in for myself.
Nature has a lot of horrible deaths, don't get me wrong.
But boiling sensory wise (at least for humans) is similar to immolation (being burned alive) and it unfortunately takes longer to actually kill the person than burning them alive.
Tigers naturally kill prey quickly to avoid injury, often going for the neck.