attachment is the root cause of suffering, and to understand this we must first understand what attachment really means. when you do something expecting a particular outcome or result that means you are attached to that work because the result of it matters to you. however, the intensity of attachment may vary and accordingly we suffer if the outcome and results are not what we wanted. we are attached to different things to varying degrees. And if more intense is the craving for a certain outcome, more intensely we suffer if we do not achieve it.
It is because you are attached to the idea that you belong in a romantic relationship. You are attached to the idea that, even if you are content, having a partner would elevate your happiness. You are attached to the idea that without a partner your life isn't as good as it can be. Those things might be true sometimes, but not all the time.
expect nothing and you will never be disappointed
It means if you learn not to expect things from others, and just do for yourself then you will not be disappointed when the other person does not follow through
Never get too attached to anyone unless they also feel the same towards you, because one sided expectations can mentally destroy you