We are all differently framed. We are not all the same. Not all of us can bear something. What another person can bear, another one cannot bear. Some people have ability to suppress their emotions to some extent while others cannot. Some people are strong in one area but weak in some other aspects. So, our nature is not the same for everyone of us.
There are those who can endure and survive successfully if they lose their love one. They are those who can't. Imagine people who easily fall into adultery even when they partners are still alive. Such people won't be able to stay for long without a spouse. So, it is not a sin or wrong for someone to remarry after he or she loses her partner.
Marriage was initiated by God for companionship and helpfulness. God said that it is not good for a man to be alone. Loneliness can kill. Someone life expectancy can increase if they he has a companion with him. So, those who want to remarry after losing their spouses can do so. It is right to remarry after the death of one’s spouse.