Sanctification, in essence, is the process by which a person whose heart has been changed by the Holy Spirit is made more like Christ every day until His return or their promotion to heaven. The process is always the believers struggle with sin, empowered by the Holy Spirit. I have found John Owens Mortification of Sin to be a very good resource for understanding this. There is always going to be the conflict between the spirit and the flesh (Romans 7). It involves the spirit putting things to death, so there is always going to be suffering, as things which we hold dear are killed, either by us or by God and usually both. There is our part in this which is the whole "take up your cross"; a wilful action to take the hard road (actually I think that 2 Corinthians 4 passage is very insightful with regards to this stuff). Also, the bible talks exclusively about God giving us trials for our growth, if we are His children. Jesus himself says that "every branch that is going to bear fruit needs to be pruned"
From my understanding suffering is integral to sanctification. Romans 8:17 tells us that we are heirs with Christ and gives us the condition for being Christ like: suffering.I remember Ivan Bowden talking about the Christian life being a cruciform life, because it conforms to Christ who bore the cross in his own body. We are in union with Christ and members of His body and it follows that we will be conformed in suffering to his image. If Christs life was marked by suffering, humiliation and rejection, then the Christian should not be surprised by these things in their life, and should even take comfort in them (1 Peter 4:12-13). Paul talks about this a lot:
“For just as we share abundantly in the sufferings of Christ, so also our comfort abounds through Christ.” (2 Corinthians 1:5) “always carrying in the body the death of Jesus, so that the life of Jesus may also be manifested in our bodies. For we who live are always being given over to death for Jesus' sake, so that the life of Jesus also may be manifested in our mortal flesh.” (2 Corinthians 4:10-11) Philippians 2:5-11 (my favourite passage in the whole bible) shows us that Christ is the human model for what we might call moral and vocational sanctification, that is in his character and in his outworked life. “…that I may know him and the power of his resurrection, and may share his sufferings, becoming like him in his death, that by any means possible I may attain the resurrection from the dead.” (Philippians 3:10-11) In this passage, Paul equates deeper knowledge of God with the sharing of the sufferings of Christ. The only way a person obtains resurrection from the dead is by being perfect, which is the goal of sanctification. There’s no question about its completion. “Now I rejoice in my sufferings for your sake, and in my flesh I am filling up what is lacking in Christ's afflictions for the sake of his body, that is, the church…” (Colossians 1:24) This verse shows that God will continue to make himself know to the church in this manner. Martin Luther was pretty clear that it was integral for sanctification:The holy Christian people are externally recognized by the holy possession of the sacred cross. They must endure every misfortune and persecution, all kinds of trials and evil from the devil, the world, and the flesh . . . by inward sadness, timidity, fear, outward poverty, contempt, illness, and weakness, in order to become like their head, Christ.It seems that the primary way Christians will experience suffering is as the inevitable and direct result of their desire to be holy. As Eric Mason says, suffering is the superintendent to the building project of our spiritual lives. “To this end we always pray for you, that our God may make you worthy of his calling and may fulfil every resolve for good and every work of faith by his power, so that the name of our Lord Jesus may be glorified in you, and you in him, according to the grace of our God and the Lord Jesus Christ.” (2 Thessalonians 1:11-12) God making them worthy is very clearly sanctification. They are being made worthy of the calling of Christ. We know from the context of the book that the Thessalonian’s situation is one of trial and persecution. Paul's prays for faith based works produced by suffering situations (the human element to the 2 pedal systems of sanctification). There’s a book by Thomas Case called “Treatise on Afflictions” that I found very helpful on the topic because it systematically steps the reader though every biblical idea involving suffering:
As Gods children, he often puts us through suffering because he loves us. Hebrews 12:9-10 talks about God doing this to make us holy, which is sanctification.