"And Esau was forty years old when he took to wife Judith the daughter of Beeri the Hittite, and Bashemoth the daughter of Elon the Hittite: which were a grief of mind unto Isaac and Rebekah." Genesis 26:34-35
We grieve for many reasons. Isaac and Rebekah had a grief of mind over their son's choice of marriage. Esau was going against the God of Isaac and Rebekah when he married into these families. The Hebrew word for the phrase "grief of mind" is actually morah. The word can mean bitterness, trouble, grief, or sadness of spirit. This is all the passage says about their grief. We know that the next chapter begins with Isaac trying to give Esau the blessing, but Esau ends up selling his birthright to Jacob out of his lust for momentary satisfaction. Esau made some poor decisions. Prior to Esau's marriage, God had blessed Isaac, after Esau's marriage, and their grief, we see that there is an undertone of disunity in their lives. We see the deceit of Rebekah and Jacob against Isaac.
How we handle grief matters. The grief of mind that Isaac and Rebekah experienced over there oldest son seems to have changed them. Perhaps the grief was weighted heavily with bitterness that rooted and turned their hearts against each other. Whatever the case, the picture is not a good one when Jacob deceives Isaac, and Esau must leave.
Whether our grief is over our sin, someone else's, unfulfilled expectations, or health, we have to keep our mind where it needs to be. We must keep our minds stayed on Him. Isaiah 26:3 says, "Thou wilt keep him in perfect peace, whose mind is stayed on thee, because he trusteth in thee."