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, social norms and whatnot. They are just being manipulated by those all the time, unless they're very mindful of what's happening in their mind temporarily. Realizing this you can't really hate anybody, and in fact you realize they are 100% like you.
> the belief in rebirth is not needed to practice metta
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.
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, social norms and whatnot. They are just being manipulated by those all the time, unless they're very mindful of what's happening in their mind temporarily. Realizing this you can't really hate anybody, and in fact you realize they are 100% like you.