"Your relationship should be a safe haven" suggests that a romantic partnership should provide comfort, security, and refuge from the stresses and difficulties of life. Not a battlefield" implies that a healthy relationship should not be characterized by constant conflict, strife, or emotional turmoil. Instead, it should be a place of peace and harmony.
"The world is hard enough already" acknowledges the external challenges and difficulties that individuals face in their daily lives. It implies that a loving relationship should be a source of solace and support in navigating these challenges.
In essence, this quote encourages couples to create a nurturing and peaceful environment within their relationship, where they can find comfort and support in each other's arms, especially in a world filled with its own hardships and trials.