Probably because you and your partner don't have the skills to make it successful.
I'm willing to bet that no one has ever taught you how to be in a relationship, and maybe the only relationships modeled for you were full of dysfunction. As result you have no idea what you're doing. The more you fight to make things better with your partner, the more distance grows between you. You long for closeness, connection, and communication, yet anytime you try to acquire these traits, it ends in a big fight. Can it really be that hard to have a healthy relationship? Other people have healthy relationships, don't they? What is their secret? Is there something wrong with me that's preventing my relationship from succeeding?
How can counseling help?
Fortunately, connection and communication are common goals for couples in therapy. But, unfortunately, therapy still has a social stigma that often prevents couples from coming until things have gotten out of control. Many people still believe that counseling is for mental health diagnoses and is exclusively for people who have something "wrong" with them. It is a place people go to fix themselves.
Often people don't realize that therapy is an inclusive setting for anyone wanting to learn and grow. A therapist will not fix your problems or demonize you for mistakes you've made but rather will collaborate with you to help you learn new ways of doing things. It is a learning opportunity to try new skills and practice new ways of interacting.
Many couples come into my office as a last resort. They've fought so hard for their relationship but continue to be stuck in their old ways of interacting. When I ask my clients if anyone has ever taught them how to be in a relationship, they always answer 'no.' I can see the relief on their faces when they realize they are no different than anyone else! It's not that there's something wrong with them or they are flawed beyond hope, but instead, they were simply never taught how to do this whole relationship thing. This also means there is hope that they can learn how to create a healthy relationship together and find the connection they've been desperate to discover.
Here are some skills couples counseling can help with:
Giving and receiving love
Setting healthy boundaries