People in a relationship, no matter how much they disagree on issues, always have options. They may be able to talk out their issues, or they may need counseling to help them come to a compromise. Even couples who have agreed to go their separate ways have options, and they may be able to reunite in a few years. Death takes away options, and it is, sadly, the ultimate end to any relationship.
In many long term relationships, death occurs when partners have been together for decades. One partner may have a disease which allows them to linger, and they can discuss the future with the partner they will be leaving behind. One of the issues with this type of relationship end is the expectation the remaining partner needs less time to grieve. Loss is still loss, and each individual needs to be able to process it in their own time-frame. Their sadness is often quite profound, and it may never disappear entirely.
Sudden death is a shock to the partner left behind, and it compounds their feelings of grief and loss. Many people question why this has happened, and they need support to get past the tragedy. They may regret the kind words or feelings they did not express to their partner, and this takes them deeper into the sadness and loss they are experiencing. Sudden loss of a relationship in this manner is generally experienced by younger people, but it can also occur to couples who have been together for decades. No matter a person's age, the sadness associated with losing a partner is deep and will last a long time.
Death is the final parting on this world for people in a relationship. There is no hope they will ever have a chance to work out their issues, reconcile or carry on their life as they had planned. The sadness of this type of relationship ending is permanent for the partner left behind, and they must find a way to carry on through their grief and sadness to find someone new to share their life.