Survival. Both there and now, and for the future - also both for self, but the species, namely offspring/children.
Certain things/features/traits are deemed desirable among the other half required for the species. With multiple competing for the same target/other half, their own place and thus dominance becomes important. With a species looking for long term relationship/partner/mate usually its a 1-on-1 type, meaning being first - by whatever means - is good.