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:
You are your own person.
You can't forget who you are, and what makes you, you. The other person in the relationship fell in love with you because of the person you were when you met. If your identity becomes strictly attached to who you are as a couple, there will be problems further down the line.
Alone time, doing what you want, helps make sure both people in the relationship don't resent each other for being stuck together all the time.
You have to enjoy the time that you spend apart.
When you do get a chance to spend you alone time, make sure you enjoy it. It should be enjoyed, because if you don't, even though you get quiet time alone, you will still begin to develop resentment.
You should never feel guilty for whatever you happen to be doing (except if you aren't being faithful). If you make sure that you enjoy yourself, you will come home feeling refreshed.