It takes a lot of work to be in a "perfect" relationship. There is no instruction manual, and it's an ever evolving situation.
When the relationship is fresh and new, it feels like you can never get enough of each other, and that every waking moment you spend apart is a waste of time. Once the relationship has grown into something more permanent, you begin to fall into a routine, but things are still new enough that problems that start rising up are easily dealt with.
But after several years of being together, things can begin to grow stale, and resentment can build up, especially if the two of you have kids. Going through the same things, day in, day out, without any variety or enjoyment has the possible side-effect of turning life into a re-run that you are being forced to endure time and again.
One thing that is important to know is that couples need to spend time apart doing things that are important to them. Just because you become a couple doesn't mean you stop being individuals.
These are a couple of things that you need to remember about spending time alone: