Not a moral stance, purely a logical one.
I want to live comfortably. I do not live independently of the world I exist in, so my ability to live comfortably depends on what's going on around me and the actions of others. So the best chance of me living comfortably depends on convincing others to take actions that result in my own comfort.
The best way to get someone to do something is to make it beneficial for them to do so. I can reasonably guess that other people want to live comfortably too, since that's what I want, though we might disagree on the particulars of what level of comfort we desire or what resources we need to achieve it. Thus, an outcome that results in both of us being comfortable is best.
Apply this 2 person solution to a society, and you create a situation where the ideal outcome is one where every person is provided for to achieve a high floor in terms of minimum quality of life, because that means I can't fall below that floor. As long as my own level of comfort is maintained, it costs me nothing for other people to be comfortable too. A society where everyone helps other people is beneficial to me, because it means that when I eventually need help I can expect to get it. Even if I am occasionally inconvenienced in minor ways to help others, this is still ultimately to my benefit because these temporary inconveniences reinforce the same framework that benefits me in return.
Encouraging people to be helpful to each other and rewarding that behavior is just a good strategy for personal comfort.