I can't say what exactly the therapist defines as positive vs. negative, but I can see how a therapist might consider it better to reinforce positive behaviors. For example, if someone is stuck in a pattern where they feel like they need to engage in negative behaviors to get care or attention from others, then it might be best not to make a big deal over it when that person is acting out negatively and try to teach them healthier ways to get the affection and attention they are seeking.