by nature, most humans are selfish because they are out for their own survival.

However, once a person has kids (or falls in love) then the concern for the "tribe" as you called it comes into play. Humans enter into agreements to live together and roam together as a means of self preservation. This at times could be I'm weaker and the only way I will survive is to be with these people. It could be something else. But preservation for species only arises once a person has children. how can we preserve this for our children? how can we make sure our children are ok? even if it's not their own children per se, this pack mentality about preserving a tribe is not an issue when it's a single person making it on their own.