Why Do We Demand Every Relationship Be Perfect?

We have so many expectations of what exactly the relationship should be. What if there is a lesson in that ship? Instead, we usualy have a mould that we've already carved out. And when it doesn't fit, we become frustrated. We play victim and all because we kind of refuse to go on an adventure. If you decide to embark on that ship with one another, why do we expect them to last as well? Some of my best relationships include ones that only lasted about a year or two.