Measurable quantities are often task/skill/work. But volunteering isn’t just a skill or task but idea of people reaching out around context of help/purpose/useful/to add to a possibility. Magnificence of the idea is many little intangibles. But isn’t it the same concept that reaches out as human instead of competitive attack on ones adding to a good idea? There was the concept of (सेवा)good samaritan that was understood by human brain. But with machines pretending to give human ideas, these concepts when measured or weaponised as in fakenews misrepresentation, cyberwar, etc lose the finer quality undermining human care. Idea of appreciation is about healthy human interaction that creates healthy participation upholding human dignity.

Assure that each one of them feel heard and that they're contributing for a great cause. When that happens, people usually feel a sense of purpose like no other.