I will not leave you comfortless: I will come to you. (John 14:18) I am with you always.
These are not merely sentimental promises meant to stir emotions. Rather, they are concrete promises of God based on the reality of Christ in us. The idea that God is floating around, comes and goes, is with us one day, and far away the next, is contrary to the Truth. We may FEEL like God is that way, but the Truth is not determined or apprehended by how we feel.
Indeed, even if we feel like God is with us, He is not with us because we feel like He is. He is with us because Christ is forever IN US.
If I am in despair and cannot seem to escape the sense, indeed, the fear, that Christ has abandoned me, or is indifferent to me, what I feel is contrary to the Truth. If circumstances seem to lie about God, the circumstances may be real, but they are nevertheless NOT telling the Truth about God. The point is, that it is normal and expected that our flesh, emotions, and soul life will not behave in accordance with the Truth. But if Christ is in us, HE is the Truth, and HE is faithful, no matter how off the track our temperament may be.
The certainty that God is with us is sealed by fact that He has bought us with the price of the Blood of Christ. If God were so much as indifferent to us, He would be indifferent to His Son. God is always with us because Christ is permanently in us.