Having someone’s best interests at heart sometimes means giving them hell. Meekly submitting to abuse by someone does them no service in the long run. Nor does trying to spare someone the consequences of their bad behavior by covering for them or facilitating their behavior serve their best interests.
What many faiths teach is to get your ego out of the way. To most people, their self is the most important thing there is, and if there’s a “root of all evil”, it’s this inflated sense of self. Some religious practices are centered on whittling the ego down to size, so that the interests of others matter at least as much as our own. Once the self doesn’t dominate one’s worldview anymore, then looking out for the best interests of others comes naturally. You’re simply not always trying to work things to your advantage; you can forget about yourself when necessary.
That’s different from a belief-driven “self-sacrificial” attitude. If you still have this strong sense of self, but your beliefs tell you that you must self-consciously force yourself to endure hardship for someone else’s advantage, that ironically becomes an ego-driven enterprise. One develops a martyr-like self-image, which is just another form of ego, and is often pathetically blind to the actual welfare of others.