Yes, the Bible does give the reason. On the principle that a skiff rowed against the stream is twice as likely to be bumped into by logs and debris as if it were floating down stream, even so, the man who strives against selfishness and sin has a harder fight. The natural tendency of the world is downward. The Christian finds as the Redeemer said, that "Strait (difficult) is the gate and narrow is the way which leadeth to life, and few there be that find it." (`Mat. 7:14`.) The Lord permits the Christian to have adverse experiences that his character may be developed. Willingness to brave opposition indicates a character pleasing to the Lord. Weak Christians become discouraged with these experiences, but strong Christians grow stronger in the Lord and develop strong characters. It is to the faithful Christian that God has given the promise of joint-heirship with Jesus in His Kingdom. To such He says, "We might through much tribulation enter the Kingdom of God." (`Acts. 14:22`)