Why does God allow suffering?

God allows suffering because trials and tribulations permanently expand our capacity for peace and joy.

Only through trials and tribulations is the human mind inoculated against the suffering that they cause. Eventually a natural immunity to suffering arises and the mind can be at peace under most any conditions.

Suffering continues until our minds become trained to cease responding to adverse conditions with adverse emotions.

All those that suffer will be proportionally compensated for this suffering with greater peace and joy.

December 14, 2018 update:
As my great friend Rob told me tonight, (and I immediately agreed) God also allows suffering so that people can learn empathy.

Empathy is the most important aspect of love and love is the most important aspect of righteousness.