I don't understand the concept of marrying somebody after your partner dies?

If I was really in love with a girl and we got married, I probably wouldn't even look for another person to be with if she dies. It just seems wrong and unethical. However, based on what I see on TV and just observation in general, it is considered as "moving on". That life goes on and generally what should be done especially when one's partner dies at such a young age. Why is that? Does that mean that you never loved your other partner before to be forgotten just like that? I mean, it's one thing to constantly mourn about the dead. But marrying another?