I think that the culture today is a huge part of this type of behavior, not just young people themselves.
First of all, people are used to choices. Companies that don't offer hundreds or thousands of choices don't last very long. No one wants to stick with the same thing forever. If you could choose only one food to have every day for the rest of your life, what would it be? If you could shop at only one store, which one would you pick? If you had to listen to only one song until the end of time, what song would be on your very singular playlist?
Likewise, if people are used to having a hundred different choices just for dinner, for many, the thought of choosing one--and only one--person for the rest of your life can be quite daunting, and, let's be honest, most people see this as boring.
They ask themselves, "Why would I have to limit my romantic choices to just one person when there are billions of others out there?" In some ways, I think Christians are even worse in some respects (hear me out) because they believe that God sure wants them to be happy, have the perfect relationship, and has The Perfect Person out there. If something, anything, or even nothing goes wrong (boredom sets in), they then decide that surely God has "Something Better for Me" out there and it's time to move on.
Second, I also don't believe it can be entirely blamed on the young people themselves. How many people do you know today who are unmarried and living with a partner, or single with kids and going through an endless string of relationships? (I'm speaking of a non-Christian situation here--this is not at all or in any way meant to disregard or disrespect our hard-working single parents who are obeying God's commands when it comes to dating.)
My point is that if the young people of today see their own parents living with someone and hooking up, what else would we expect them to be doing?
People talk about "the good old days" when people stayed together, and I do think part of that is true. But I also think we're seeing a snowball effect from a cultural attitude that has been decades in the making. "Back in the day" people stayed with each other, but it was often out of obligation or because there was no where else to go (divorce and breakups were much more socially taboo). I highly suspect that many people of those days (not all, of course, but many) probably would be doing the same things as today, it's just that back then, it wasn't as socially "acceptable".