It really depends on the people involved in a relationship.

Love and Lust can get very crossed. Some people will feel like they are in love, but are confusing it with lust, and some will feel they are only in lust with someone, when they are really in love.

Where your master says that love can be dangerous to an M/s D/s dynamic is when a Dominant who exercises their sadistic inclinations on someone they are not in love with. The problem starts when they fall in love, and then can no longer harm the loved one because they have a conflict with wanting to protect cherish and build up their partner and wanting to hurt and humiliate their partner.

This conflict is a problem unless they are able to reconcile it. If they can't, either the SM side of the relationship will fall by the wayside, or the love will. Sometimes the Dominant will end the relationship due to this inner conflict. It usually leaves the submissive confused and hurt, thinking they did something wrong.