To put it another way, as long as we are experiencing a defect like jealousy or envy, we cannot experience loving-kindness. And when we are experiencing loving-kindness, we cannot experience jealousy or envy. The two cannot happen at the same time. To be jealous is to desire someone else's well-being and success for yourself. To experience loving-kindness, on the other hand, is to be happy for others and rejoice when you witness their well-being and success, whether it be of a material or a spiritual nature. Jealousy runs completely counter to this disposition. In a similar fashion, when you experience genuine compassion, you cannot simultaneously experience hatred, anger, or aggression. As long as there is the one, it will displace the other.

-- Khenpo Karthar Rinpoche