Altruism is a human impulse. It feels good to be able to help someone in need. It's beneficial for the species as a whole when we help each other so one could say there's evolutionary pressure in that direction.
While it feels great to help someone, it is an outrage to have one's generosity taken advantage of. We might not mind giving money to a homeless person. If, however, we discover that person isn't really homeless but making a good income from panhandling, it burns us up.
Then there are people who truly need help, but helping them never seems to solve the problem. Some people are able to absorb a nearly infinite amount of aid without ever bettering their circumstances. Giving them a hand is a never ending labor of Sisyphus.
Christianity teaches that we should give the shirt off our back. That's all well and good, but what happens when the person we give the shirt to loses it or doesn't take care of it? How about if they sell it?
Then there's the prepared person. They don't have to give the shirt off their back. They have other shirts they can give. If that shirt goes to waste . . . well . . . it's not the only shirt they had. In fact, one can be more generous if they have a lot in storage. If you've judged someone's character wrong it's not the end of the world. There are other shirts to go around to those truly in need.
Of course, the real world is more complicated than that. There are people I'll give certain types of assistance but refuse other types of aid. We all know about giving booze instead of money so they won't squander it on food . . . or is it the other way around . . . depends on the person, I guess. There's one person I'll help with any number of things but will refuse to move them ever again. After moving an ancient heavy refrigerator up three flights of rickety stairs they decided to move again three weeks later. That move, and every move that person made since was without me. Even Job had limits.
Still, helping people might sometimes be a waste, but it's the right thing to do. Sometimes a little help at the right time can be life changing. Of course, you can't help someone if you yourself need help.
-Sixbears