Destroy you and leave you without anything. Nobody wants to take along a complainer. Nobody wants to promote a complainer. Nobody wants to live with one. Nobody wants to be a partner of one.
Nobody wants to have one around.
Complaining leaves you out of more opportunities than you can possibly imagine if you let it take over and grab you by the throat. If you don't think complaining is bad ask the children of Israel of Old Testament fame. They are typical of us all, but their story just happened to get in the Book. The story says children of Israel are slaves in Egypt. God performed a series of dazzling miracles and gets them out.