The value of something?

What decides the value of something? Is it societal pressure as we see with diamonds and gold, or is it increase in living condition as with a new house? Is it set by supply and demand, as in the cornerstone of a market system, or is it purely sentimental and arbitrary? Maybe its a bit of all of them.
I value some family pictures, a few gifts, and trinkets more than things that hold a much higher monetary value. On the other hand I have things I valued a lot in a different time. When I first purchased my Zippo lighter, an original not a knock off I felt like it was the coolest thing ever, now it mostly collects dust. Not because its a bad lighter but because its just so much easier and more practical to buy disposable.

Anyway with values attached to everything, we've kind of blocked out when it comes to ourselves. It's considered a faux pas to show or say anything that gives the impression that you see people as having a value just like a watch or book. It is quite true though, for me anyway. You see while most people tend to pretend like they don't notice, they distance themselves from people who have low value to them, and gravitate towards people who have high value. If you have a person in your life that just takes from you, or that you don't enjoy spending time with, you tend to not spend time with that person. There can be other reasons, I don't go to visit my grandfather in the home much because;
1. He is senile so he doesn't remember if I was there, or even who I am when I go there, and when I do he sleeps most of the time.
2. It's excruciating to watch a person you look up to, who has been a person you saw more or less every day of your life until you were 18, turn into a shell of his or her former self. Watching a person become just a body, like their personality being ripped away is more painful than them being dead before hitting the floor.

Sorry for the short tour through sobville, I wrote that because it illustrates that sometimes you avoid a person not because of a lack of love for them, but to take a path of least pain. Now if you have a person who is improving the quality of your life in it, then you tend to want to keep them. Now the value of a person is relative, in the land of the blind the one eyed man is king. It can be having the newest shoes and clothes in a subgroup of fashionistas or the Sheen brother without a rap-sheet when they need to explain the dead hooker.
Regardless of that, we all assign values to people. I am honest about doing so and while it does sometimes lead to regretful situations and probably a result of our development.

Among a species the goal is to survive and reproduce, any successful species has to do this. In a world where I am programmed by nature to get my DNA to survive, or to use the literal term "blood" as its used in historical texts, assigning values makes sense. The closest thing to my DNA is the DNA of my siblings, my cousins, and so on. So in order to make my families DNA survive it does make sense to protect them, and to help them. Now a single family isn't much of a force and doesn't offer diverse enough DNA so add a few more families and you have a small community. Now the people of this community will value a member of their community over an outsider unless that outsider offers them something more worth than their members.
Sacrifices to the Gods in ancient and to a degree modern times is an example of a social group valuing the goodwill of their God(s) over members of other groups and in some cases even their own group.