You get married and have two children. Then your spouse files for divorce. You split up, wait a few years, and get married again. Your new spouse also has two kids from a previous marriage.
This relationship lasts. As you both grow older, you decide to do your estate planning. Can you keep it simple and just leave everything that you have to your spouse? Some people call this an "I love you" will. It shows your dedication to your spouse and helps take care of them when you're not around.
You can do this if you want, but you have to realize that means your children from your first marriage may not get anything. Your spouse is not obligated to include them in his or her will. As a result, if your spouse passes away 10 years after you, they could leave all of their assets -- and yours -- to their own kids from their first marriage. Your children get nothing, even though you wanted them to.
The best thing you can do to avoid this is to pass your assets on to your own kids when you pass away. Take care of them up front. Make sure that your spouse cannot cut them out of the inheritance. That's the only way to guarantee that something goes to them.
Of course, what you decide to do is up to you. Every situation is different. The key, though, is to really think about the future and consider all possible outcomes while you look into the estate planning options you have.