This article is from the Polyamory FAQ, by email@example.com (H. Wilper) with numerous contributions by others.
Some people seem to have no jealousy; it's as if they didn't get
that piece installed at the factory. Others, including some
long-term polyamorists, feel jealousy, which they regard as a
signal that something needs investigation and care, much as they
would regard depression or pain. Jealousy is neither a proof of
love (and this is where polyamory differs from possessive or
insecure monogamy) nor a moral failing (and this is where
polyamory differs from emotionally manipulating one's partner(s)
into relationships for which they are not ready).