This is a simple application of the UP/DOWN Metaphor frame.
As it says in the link above:
English speakers (like all humans) are oriented vertically with respect to a gravitational field, so the up/down dimension is significant, and English uses it in a variety of metaphor themes. All of them are coherent, i.e, we tend to think of them in the same ways (e.g, LESS, SAD, WEAK, PASSIVE, and WORSE are all negative evaluations, and vice versa.)
a) UP is MORE (DOWN is LESS):
- The prices are rising/falling.
- The stockmarket’s moving up/crashing.
- Turn the volume up/down.
b) UP is HAPPY (DOWN is SAD):
- He’s depressed.
- feeling really up/down about it
- What a downer!
c) UP is POWERFUL (DOWN is WEAK):
- upper/lower classes
- the highest levels of the government
- oppressed masses
d) UP is ACTIVE (DOWN is PASSIVE):
- The computer is up/down.
- Are you up for some handball?
- Rise to the occasion.
- Down in the dumps
e) UP is BETTER (DOWN is WORSE):
- higher/lower animals
- He fell down on the midterm.
- a rise/fall in performance
- aim high
f) UP is ABSTRACT (DOWN is CONCRETE):
- He’s got his head in the clouds.
- He’s got his feet on the ground.
- Come back to earth.
- higher mathematics
- high-level cognitive functions
- low-level details
- new heights of abstraction
- down-to-earth solution