Why does my councillor think dissociation is bad for me?

Not bad or good: Dissociation happens as an unconscious defense mechanism in order to protect the psyche from unbearable pain. It is not bad or good, necessarily. But when large portions of reality are split off or disconnected, it makes it harder to make reliable decisions in the day-to-day world. While dissociated, you can expose yourself to danger, even though your unconscious mind thinks it's protecting you.

Hard to say: without knowing what s/he said. Dissociation is an ability we all have to separate aspects of our psyche that are normally smoothly integrated. This allows us to disconnect from feelings that are overwhelming so we may continue functioning without the interference. An example is going emotionally numb. It can become a problem, e.g., by keeping us from knowing what we feel. Ask for clarification.