The word up usually has a positive connotation - thumbs up, look up, go up in life - whereas down usually has a negative connotation - look down, go down etc.
Why is this so and when did such an usage begin?
I did read that up comes from the word for uphill and down comes from the word for downhill, but it still does not explain the connotation assignments.
Probably because most of the "good" stuff is traditionally higher/above them. Positions of power (kings, priests, gods) have usually been placed up high (altars, daises, mountaintops, pulpits). Birds, trees, fresh air, rain, light, all that comes from above.
Meanwhile, down below, you have dirt and muck and filth, worms and insects, trash, darkness. When disaster strikes, it is usually more dangerous to be further down--you'll be crushed if something falls on you. And there's the fear of the unknown: we can see through the air, but not into deep water or into dark caves (before artificial lighting).
To start with, Up/Down is a Metaphor. One of the big ones.
And that means it's got lots of uses.
All the ones mentioned in answers here are explained on the second page of this handout.
For further details, see this article, and the works it references.
Metaphor is not just a Greek word; it's the real deal.
Metaphor is how we think. And talk.
"Up" is often associated with more of something, since if you are looking at a pile of objects, the more there are the higher up the top of the pile will be. And more is often considered better.
But with verbs like "look down", I think it has a more literal sense: a child "looks up" literally to adults, for example, so someone you look up to is someone you admire or are impressed by. This is like the metaphorical usage of "giant" to mean "a great or important person".
The assertion that up and down map to good and bad respectively isn't always true. As one example: plans can be "up in the air", meaning still uncertain, and they "settle down" as the decisions become more definite.
Beware of oversimplifying. Context matters.
(Sign in a software lab: "The uplink is down, but the downlink is up.")
I can only imagine it relates to people looking at the stars above, e.g. the heavens and gods that ruled them above, and you go "up" to a better place when you die. The opposite is just down, e.g. hell if you sinned.
