It is a mystery

It's a fine impulse; you have much and feel you should share. Generosity is a virtue.

It's Justice that's blind, though, not Virtue. (Virtue doesn't generally get a single personification!) And I get to quote Aristotle -- "at the right times, about the right things, towards the right people, for the right end, and in the right way, is the intermediate and best condition, and this is proper to virtue".

So just like you aren't obliged to be kind to someone who is being mean to a dog, or do something brave when there's a prudent option right there, though kindness and courage are surely virtues, I don't think you're at all obliged to be generous just because someone is wanting at you. It's more than OK to be looking for the right people to be getting into a gift network with.