Why did Peter ask christians to rejoice inspite of the trials they may be going through?

Peter exhorts Christians
to rejoice in the hope of the glory which awaits them owing to the resurrection
of Jesus Christ. Although in this present life, their joy might be limited by
the trials and persecutions that would come their way, such trials and
persecutions are meant to prove the genuineness of their faith and make them to
reap the rich reward of salvation when Jesus is revealed.

In order to fully realize the hope of salvation, Christians should
live holy lives in the full knowledge that God, the
impartial judge would take note of their sufferings
on the day of judgement. Furthermore, they should not forget that Jesus himself
passed through greater suffering. If they were reproached for the name of
Christ, they should count themselves blessed. They should, however, not suffer
as murderers, thieves, or for any wrong doing but if they suffer for Christ,
they should not be ashamed. They should instead, glorify God for on that day,
judgement would begin with those who call upon the name of God and the fate of
the unbelievers might be terrible.

Some of the virtues for a Christian to
manifest inspite of trials are: 

• They should not conform to the carnal
  desires of men of the world, 
• They should remain sober and hopeful of the
  glory and salvation that would be theirs when Jesus would be revealed (at his
  Parousia). 
• The fear of God should be their watchword because God is an
  impartial judge, 
• They should remain forever in their Christian faith,
  unshaken by any wind of persecution, the affliction of the enemy or Satan, the
  devil, 
• They should always praise God both in time of sadness and joy.