Apart from ordinary niceness, I can think of three examples of small voluntary contributions people can make to society. Each of these contributions accomplishes very little on its own but in all three cases, millions of contributors in unison can make a difference.
The first example is voting. In reality, your vote is only very slightly better than worthless. But thousands of votes together start to matter a little bit. And millions of votes in one direction can matter a lot. So even though the world will be no better or worse if you vote or if you don't vote, if everybody votes, the world will probably be better than if most people don't vote.
The second example is recycling. In reality, your recycling accomplishes almost nothing. All the waste produced by all the homes in the world put together accounts for only a tiny percentage of the world's waste. But recycling still accomplishes a tiny bit of good and if everybody recycles, that can amount to something kinda sorta substantial.
The third example is vegetarianism. Boycotting the meat industry for the rest of your lifetime can save a certain number of animal lives - and that's nice. But given the vast amount of animals still living in torture and the fact that most of your peers will continue to eat meat, you aren't really accomplishing much in the grand scheme of things. You are, however, contributing to a good cause, playing a microscopic role in some larger movement. Mainly, you are signalling your interests and values. One vegetarian alone isn't accomplishing much, but if everybody went vegetarian, that would vastly improve the state of the world.
All three of these situations require one to decide whether to play one's tiny altruistic part in a massive prisoner's dilemma or whether to say "fuck it" and worry about things they can actually control.
Are you consistent across all three? If you think going vegetarian is a waste of time, do you have the same attitude about voting and recycling? If you're adamant about the importance of voting, are you equally adamant about recycling and vegetarianism? What is the crucial difference?
I think all three of these are Good Things but I don't think too much emotion should be invested in any of them. If you forget to recycle something, too bad. If somebody you know doesn't believe in voting, who cares. If your friend refuses to give up meat, well whatever. You're playing a long term game to minimize expected suffering - it pays to be forgiving of small differences and offenses.