[TimTheTinker]

A Christian theologian would explain it this way: the point is what rewards the giver is seeking, because that is what reveals what’s in their heart as they do the outward action of giving. As one author observes, “The worth and excellency of a soul is determined by the object of its love.”

A gift given in secret is seeking the rewards truly worth having: the praise that comes from God. Those who seek that find it, because they loved what was truly worthy of love. But gifts given for others to see are seeking a reward on earth, not heaven—and that’s what they get. Either way, they’re not penalized if, say, the word gets out against their wishes. That’s not the point: God knows and judges the heart and rewards them based on what they were actually seeking.

So seeking rewards is not only categorically OK, it’s actually encouraged, but with the caveat that the seeker set their heart on what is eternally worthy of their love and desire.

[IfOnlyYouKnew]

That explanation doesn't require theology. I've seen Peter Singer make the case that it's only "true altruism" if you're just as happy when someone else does something good as when you do so yourself.

[TimTheTinker]

Sort of, but the difference is that Christian altruism results in an increasing expectation of being praised and rewarded by God on the other side for being his agent of love to others in this life.

The result is a lot of joy, knowing one has brought blessing and joy both to God and to other people.