A warning about growing bitter appears in Deuteronomy; we are told that if we do, we will grow poisonous spiritual fruit.  This warning is echoed in Hebrews and Acts.  We can become bitter because we have done things that we know we should not have done.  We can become bitter quite innocently, simply by failing at something very important to us.  Either way, bitterness is toxic.  Unlike anger and envy, bitterness digs in deeply.  It makes us seethe.  It becomes long-lasting, far outliving whatever caused it.