26 Be ye angry, and sin not: let not the sun go down upon your wrath:
27 Neither give place to the devil.
28 Let him that stole steal no more: but rather let him labour, working with his hands the thing which is good, that he may have to give to him that needeth.
29 Let no corrupt communication proceed out of your mouth, but that which is good to the use of edifying, that it may minister grace unto the hearers.
Bitterness keeps you from loving (the LORD and others), and gives the devil a foothold in your life/heart. If you don't forgive, the serpent gets his fangs into you