The person leaving may need permission to enjoy what comes next.

Guilt attaches itself to departure easily. Someone moving on often feels that visible happiness would be an insult to the people staying, so they keep the excitement quiet and the goodbye becomes heavier than it needed to be. What they are waiting for is a sign that being glad is allowed. Nobody asks for that sign, because asking would sound ungrateful.

Tell someone who is leaving that you hope the new thing is good, and mean the whole sentence. Ask them what they are looking forward to, not only what they will miss. Missing each other and wishing each other well can happen at once.