Jealousy in romance is like salt in food. A little can enhance the savor, but too much can spoil the pleasure and, under certain circumstances, can be life-threatening.
We envy only those whom we feel ourselves to be like; we envy only members of our reference group. There are few successes more unendurable than those of our close friends.
Only those beneath me can envy or hate me. I have never been envied nor hated; I am above no one. Only those above me can praise or belittle me. I have never been praised nor belittled; I am below no one.
There is a proper measure in all things, certain limits beyond which and short of which right is not to be found. Who so cultivates the golden mean avoids the poverty of a hovel and the envy of a palace.