Are friendliness and loyalty important ethical factors? Is it more ethical to cause suffering to an unfriendly and disloyal person than a friendly and loyal one?
What about plants? Do they enjoy being uprooted and eaten? Well, there is no chemical process that we call "suffering" so it must be okay. But, these mammals you see, they're so close to us! And we don't like it when humans suffer. Even worse, most people hate when something cute suffers, even more so then a human.
There is no real greater ethical meaning here, just arbitrary lines drawn in the sand by people with strong feelings one way or the other. And yes, the same goes for the suffering of us humans.
In either case, if plants did feel pain, would a person cause more suffering by eating plants or by eating animals that eat plants?
If at some time in the future it can be proven that plants feel pain, then it will depend upon their level of pain as they do not have a central nervous system nor brain as animals/insects.
Everyone will have to decide for themselves what course of action to follow, as we do need to consume food in order to survive.
Star Trek replicators would solve all these philosophical quandaries.
Deep question. Seems like it's usually more of a postulate of ethics than a logical conclusion. Treat others as you'd like to be treated, for example.
If you're looking for a scientific answer, it's probably that empathy makes you suffer when you see it in others. And empathy is instinctive behavior that presumably evolved for the good of the species. Though evolution might be "selfish" in its own way, preferring to protect some species which are useful to us in other ways while eating others.