Caring more about one or the other is exactly the issue. The reasoning about the degree of which to care about either at all is the issue that cannot be resolved objectively.
Putting a dam in place that won't impact the ecosystem so much--sure, we can minimize the tradeoffs, but the tradeoffs remain, and all resolve to what we value vis a vis what it costs. Which is not objective.
Think of it as akin to mathematics. Mathematics starts with axioms, which can be completely arbitrary, and then derives an entire universe from them by following only objective, formal reasoning. Similarly here, you have some personal values that are (to some extent) arbitrary, but accepting those, you get to use objective reasoning to devise a set of tradeoffs that satisfies those values the best.
Also sometimes different people with different values aren't really in conflict, they just don't have enough knowledge to realize their values aren't being violated. Fix that, and you can get agreement.
Seriously, a better way would be to start doing 5-whys on this, instead of going for "non-objective things". A quick attempt will tell you that we care for both ecosystem and power, but depending on the local context, we might care more about one or the other. Maybe we can import power from elsewhere, or put a dam in a place where it won't disturb ecosystem so much. Maybe the ecosystem can handle the dam, or really the power is desperately needed.
You can quantify this all if you bother. Problem is, most people don't bother, they very quickly stop thinking about it, and start going by their gut.
(I'd say, they stop behaving like adults, but since a lot of adults have this problem, I guess we need a different phrase.)