How can I be happy at other's happiness?

How can I be happy at other's happiness ? How can I develop my willingness to see others' being happy ? If I define a happiness level, scale of 0 to 100 about being happy at others' happiness, I think I'm at the 25-50 level. How can I promote it to 90-100 level ? Sometimes, I do loving-kindness meditation. But I'm looking for some kind of understanding / wisdom about being happy ? How can this help in path to the enlightenment ? Or Should I do loving-kindness meditation to gain that wisdom ?