We should be joyous in the sense that our own death means the prospect of our going to be with Christ (2 Cor 5:8; Phil 1:21-23; Rev 14:13). We need not fear death - the final enemy - as even it is incapable of separating us from the love of Christ (Rom 8:38-39). Christ’s death delivers us from any fear of death (Heb 2:15).
Nevertheless, our sorrow should be mingled with joy and hope (1 Thess 4:13). Notice that Paul does not say there should not be grief - but that our grief should be different from the grief of an unbelieving world. Though we may grieve, we can also take great comfort (1 Cor 15:55-57). Worship can be especially important at this time in one’s life ... i.e., David and the death of his on (2 Sam 12:15-23); Job and the death of his children (Job 1:18-21).
Regarding the death of unbelievers, our sorrow and grief is not mingled with the joyful assurance they have gone to be with the Lord. This sorrow, especially regarding a close loved-one is very deep and real (Rom 9:1-3). Yet, we often do not have absolute certainty that an individual has persisted in refusal to repent all the way to the point of death. The knowledge of impending death often results in genuine heart-searching on the part of the dying one. At such times, sometimes the words of Scripture or testimony heard long ago are recalled and the person may experience genuine repentance and faith. Sometimes we do not know. Nevertheless, after a non-believer has died, it would be wrong to give any indication to others that we think that person has gone to Heaven. It is misleading and gives false assurance, and may diminish the urgency of the need for those still alive to repent. It is better to focus on the fact of the sorrow we may feel at the loss causes us to reflect on our own life and destiny. It often, is the best time to share the gospel, as the Lord presents opportunities. It is often helpful at such times to speak with genuine thankfulness for the good qualities and the good things the deceased has done. We can thank God for the common grace exhibited in the individual’s life. This requires honesty and mature judgment (the inevitable reaction to the death of someone widely known as evil and destructive [ie, Adolph Hitler]; Prov 11:10). David speaking of Saul despite the fact that Saul has tried several times to kill him (2 Sam 1-19-24)