First of all, the only one who can adequately answer this question is God and on some matters He has chosen not to fully reveal His purpose for allowing such things. The short answer is that God expects us to trust Him and to respond in faith to what He has revealed.
This may not be the answer that you were expecting or hoping for. You may find yourself bitter and blaming God for something that has happened and want / need more of an explanation. Admittedly, sometimes our best attempts to explain God's plan and purpose and what He chooses to allow just don't seem to fit or satisfy and we run the risk of doing more harm than good. Recognize this as you consider the future posts. Also, please consider all the information presented before deciding on your response.