Empathy is at the core of the moral life. Without empathy, a person is unable to make the mental and emotional transference to the other.
For example, a psychopath is unable to feel the fear or the pain they are inflicting. Without empathy, we become self-centered to the extreme. We have no concern for the other. We cannot share their joys or their pain.
But, empathy comes with its burdens. Yes, we rejoice in the happiness of others. Their joy lightens our lives. It is the pain that sometimes becomes too difficult to bear.