Why did Christ have to die and suffer to become head of the Body?
Christ had to die and suffer to become head of the Body because His headship over believers is pictured in marriage, where the husband is head of the wife and should be willing to die for her. He suffered and died for us as members of His Body, though the resurrection overcame that death.
Les explains this from Ephesians 5, where the marriage relationship is likened to our spiritual relationship with Christ. The husband is head of the wife, and Christ is head of the Church. For Christ to become the head of you and I as members of the Body, He had to die and suffer for us. The power of the resurrection overcame that death, but nevertheless He suffered and died for us as members of the Body. By the work of the cross believers are now under grace, not under law; they are become dead to the law by the body of Christ, so they should be married to another, even to Him who is raised from the dead. As soon as we become a child of God, we are literally unioned with Christ, just like a husband and wife in the marriage relationship. That same headship means a husband should be ready to die for his wife if necessary.
For the husband is the head of the wife, even as Christ is the head of the church: and he is the saviour of the body.