Like nearly all Buddhist teachings, there are practices you can do to develop these faculties. For example, you can reflect that all beings have at one time or another been your mother, or your child; realizing this, how can you feel hatred towards any other being? It doesn't work for me, of course; I don't believe in rebirth.
Imo the belief in rebirth is not needed to practice metta, you just need to realize that every being wants to be happy, just like you. Yet all of them are mired in delusion and most of the time do things which go the opposite way of generating real happiness, we all do. Everyone is mired in delusion and most of the time react to feelings of anger and craving (vedana), which are generated based on their history, socia…
That's not what I was saying; I was commenting on that suggested technique for developing metta, in particular, which is dependent on belief in rebirth.