Because we keep the view of self and hence of others as well. We are dualistic. We differentiate selfs and others. Also, we have little awareness that even our "enemies" are or have been kind to us, hence we feel that only those who benefit us directly are kind to us. Also, we tend to forget even those who have been kind to us before, yesterday, last week, last month, last year or many years ago. We are not grateful because we do not remember their kindness. We only focus on the immediate and marginal benefit. This can be reverse by contemplating their kindness from now, yesterday, last week, last month, last year or many years ago, all the way back to past lives and to extend to all sentient beings, then we develop loving kindness, compassion, altruism and boddhicitta in tandem. This will bring lasting happiness according to the Dharma.