No, what your grandson has done doesn't make sense -- but that doesn't make it any less tragic. Can anyone be as heartless as those who peddle drugs to our youth, ignoring the destruction and death they cause? Like the evil men of Jeremiah's time, "they are cruel and show no mercy" (Jeremiah 6:23).
Why do people turn to drugs, particularly young people? Some start using drugs simply from peer pressure; their friends are doing it. Others are looking for a thrill or a new experience, and give little thought to the risks they're taking. Still others are seeking to escape from their problems, and drugs (as well as alcohol) falsely promise relief. But whatever the reason, they usually give little thought to what could happen to them.
No doubt your daughter and her husband will do all they can to find their son and welcome him home; they love him and don't want to see him destroy himself. He needs help to break free of his addiction, and I hope your entire family will be praying that this will happen.
Above all, I pray that both they and their son (and others in similar situations who may be reading this) will turn to Christ and find in Him the answer to their deepest hurts and needs. God accepts us just as we are, and when we give our lives to Christ, He comes to live within us by His Spirit. Jesus said, "Peace I leave with you; my peace I give you" (John 14:27).