Children represent a significant time and financial investment. Parents often say that they live their lives for their children and it can be easy to see how they forgo everything else to care for their kids. The balance of this is the love for and from the child. It's a rewarding experience even though it's a lot of hard work. For people who do not have children, it can be harder to see the benefit side of things (other than 'babies are cute'). In other words, you can't miss someone you've never met. So you can't necessarily know the rewarding side of having children until you've had them.
So, some couples might decide not to have children because they like the way their lives are currently, rather than they don't want their potential children. It's all about living the life you want to lead, and in some cases that doesn't involve becoming parents. It's not a negative choice it's just different.