The limited compassion of human beings.. Most people don't care that much unless they're directly affected. This limits the willingness of society to do what's arguably in our best interests, like preventing severe environmental problems by limiting unnecessary consumption, or contributing to a culture where we help provide everyone with basic necessities like food - we may be the ones needing help in the future. I think not caring that much for others also contributes to meaninglessness, unhappiness, and mental health problems.

I don't know the solution, but am trying to "work on it" through e.g. donations & vegan advocacy - I think reducing the abuse of animals in factory farms is a "low-hanging fruit" for reducing suffering & ecological problems - it's usually much more efficient to eat plants directly instead of feeding them to animals.