May you be well. May you be peaceful and at ease. May you be happy. Saying the same words people have said for thousands of years connects us to a lineage of the heart-mind, to a long community of caring and doing, to humanity. The Tibetan word for this quality of lovingkindness translates as basic friendliness.
Sympathetic joy is a gladness, a gratitude and rejoicing in the happiness of others.
Therefore, it is a quality not to be impeded by judgement, comparing, scorn, envy or boredom. It has to do with patience, courage, integrity. This is not about detachment. The Jataka tales are hundreds of stories which are traditionally told of the many reincarnated lives of the one who was finally born around 563 BCE as Siddhartha Gautama, the Sakya prince, whom we call Buddha. A little knowledge of folk tales will make it clear that many of these stories are older tales which have been adapted into the Buddha's legend, because they are teaching tales which highlight wisdom and compassion.