^They're very efficient at working through the stages of grief in the future. Kirk can be mourning the death of the redshirt whose father sponsored him in the Academy in one scene, and then by the end of the episode be laughing and joking with Spock and McCoy. They can't muck about with extended grieving periods, they've got places to go!
I'd argue that they're in a perpetual state of denial, as no one seems to take the time to grieve. Kirk was momentarily stunned by the transporter accident, but otherwise seemed unaffected by the death of a science officer whom he recommended serve aboard the Enterprise
By the end of The Motion Picture
, everyone seems content to move forward with their lives, as the only mention of Decker or Ilia occurs when Kirk orders they be listed as "missing in action." No one seems surprised or saddened when Decker doesn't appear on the bridge with the rest of the away team, although Kirk does appear to pause as though reflecting on their loss. It may be that they were watching events unfold on the main viewer, but everyone seems to gloss over Decker's disappearance, despite his having been their captain only a few days earlier.