In the story of Musashi, Musashi has a coherent life goal of becoming the greatest swordsmen of all time. But he has a beautiful girl named Otsū with him. He loves her and she doesn't want him to move away.
The problem with Musashi is that he won't be able to work on his swordsman skill if Otsū stays with him. He needs to practice alone free of distraction.
Even in the midst of dilemma, he leaves her behind to work on his life goal. It was hard for him, so he just left her without letting her know.
He could have settled for happiness and stayed with the girl for the rest of his life. He would have been happy.
But, if you want to achieve something, you should be willing to sacrifice your happiness. That's what Musashi did. He broke his own promise to stay with her to start following his main goal -- goal to become the greatest swordsman.
From this, we can conclude Musashi's take on relationship and life goals. Due to this choice he later became the greatest swordsmen of Japan. This wouldn't have happened if he stayed with Otsū and settled for happy relationship.
Who in this world has achieved greatness just by being happy and staying in comfort? No one, and you won't either!