retyopy said:
What makes us do the things we do? Or, more specifically, what makes us NOT do things we could do? My friend is in the room right now, playing Animal Crossing. (Don't laugh, it's a fun game.) What's to stop me murdering her right now?
OP, why are you trying to find a reason to murder your friend?
Of course the reason for morality is pragmatism. No one wants to deal with your shit if you're a prick. Altruism and a pleasant demeanor strengthen interpersonal relationships with those I provide benefits for, which provides indirect benefits for me over time as I exploit these relationships and as I am exploited in turn. These relationships form a sphere of protection for me and other inhabitants of the sphere.
The currency by which you deal with the group is trust. It's good not to murder or steal, because these acts make you unpredictable and dangerous to the group; you lose trust, and therefore influence and support. Inversely, you should help a friend in need, because this demonstrates compassion; you are a person who can be counted on to be good, thus if another person does good by you it won't be a wasted investment.
Human biology and social makeup reinforce this principle through emotions like love, friendship, and obligation. These emotions can mislead you into doing things that are against your own long term interests, but in general provide a good shorthand for the pragmatic choice. i.e. it feels good to make your friend laugh,
AND THE REASON SECRETLY IS that it strengthens your bond and makes her more likely to help you if you need help or provide for your emotional bonding requirements as a human organism.
While bonding is typically enough to keep everyone in line when the group consists only of a tribe or kin group, where everyone can know everyone else personally, it can't be relied on for larger groups where familiarity between members is not guaranteed. This is the role of state, religious, and professional ethical codes. These codes provide a standard of behavior that their members are advertised to adhere to, to enable trust and a working relationship between strangers in the group.
You are now edified. You may now return to bonding with your friend to reduce the probability that she will murder you.