The word is an imperative, allowing for no other fitting response to the fiery trial. Peter’s word is the common term for being glad, rejoicing.
The command is necessary because we by default resent suffering. Welcoming suffering, rejoicing at trials, requires a shift in thinking, a decision of the mind to see suffering as something positive.
13 But rejoice insofar as you share Christ’s sufferings, that you may also rejoice and be glad when his glory is revealed.