Loneliness in marriage means the relationship stopped growing the day you said, "I do."
Wow... harsh, huh? I bet most everyone didn't expect me to put the brutal truth right up front. But, here is the thing... I didn't purpose the answer up front to knock the wind out of you and keep you down. I put it up there as an exercise of introspection and honesty.
The thing is, it is normal for relationships (formal, informal, labeled, legal, etc) to go through certain swings and cycles. However, a good relationship should look like this, a long trend of growth:
A less than healthy one... a bad one, looks like this... nothing but ups and down without growth:
So, what do we do? How do we live out healthy relationships like the first graph and not like the second? It's all about growth. If you grow together, you will grow closer. If you grow separately, you will grow apart.
Think on how much time in your day you are with or apart from your significant other. This is the start. Then think on how much cognitive time you are with or apart in your interests, hobbies, career, family, and values. Are they convergent or divergent? Certainly, not every couple can converge on the stereotypical interests of their significant others. However, there are three ways to make best the situation into something even better.
First, find a new common interest. What I find that many healthy couples develop is a 3rd interest which becomes the new them. Neither person of the couple had an interest in this third option. Yet, because this has become their new normal, it becomes part of their definition of who they are and how they spend their time together. This concept of new normal is very important.
Secondly, there exists at least ONE interest of the other which is acceptable and welcome. Pursue it. And, encourage mutual reciprocation on this. I personally have found that things that didn't used to interest me but has always been interesting to my wife are now my shared passions. The best silly example of this is behind the scenes content for movies. My wife loves behind the scenes for anything and everything. And, I didn't start out that way. In fact, I hated them at first. But now, I really enjoy them!
Thirdly, consider what is more important. Is your hobby more important? Or, is your relationship? If you've known me for some time, you know that I used to be an avid surfer. There were winters in San Diego I used to surf those waves where news reporters and life guards told you to stay away for your own safety. Those were the waves I craved. Well, after a while, it seemed that surfing only really benefiting me -- a self experience. What was more important to me was my time with my wife. My safety and my quality time with her were more important than my short time in the water. So, I gave up surfing... for now. After all, someone needs to teach my son ;) But, that's another bit for later.
Some Closing Thoughts
If you are finding yourself lonely in a relationship, remember the core of the issue isn't that the love has gone dry or you're going through a phase. What has happened is that the two of you have and are continuing to grow apart. You must take intentional, action oriented steps to mutually find your way back to each other. This must be done on all levels of life's interests: Intellectual, Physical, Emotional, and even Spiritual.
Converge on these four dimension, laboring to ensure that the beloved is upheld far above your own needs, and I can guarantee you unbound success in your relationship going forward.