There might be different reasons for different societies but Planet Money approached this question and they made a solid case.

1. It is Chemically stable.
2. Not toxic
3. Doesn't corrode
4. It is rare... But not insanely rare.
5. Low melting point

It is important that it doesn't corrode because that means it holds value over time. The rarity is also important because scarcity is useful for the value. It can however be found in rivers and streams so you don't need technology to attain it.

There is one other element that also ticks all these boxes and that is Platinum, the problem with platinum is that it's melting point is 3000 degrees fahrenheit (1668 centigrade) but the melting point of gold is a lot lower making it much easier for pre-industrial people to work with.

Silver btw also qualifies except for the chemically stable part, it tarnishes, it has however been used for currency by many civilizations. Oh and rhodium or palladium but they weren't discovered untill 1800's. It is no coincedence that these are all precious metals.

http://www.npr.org/blogs/money/2011/02/15/131430755/a-chemist-explains-why-gold-beat-out-lithium-osmium-einsteinium

http://www.npr.org/blogs/money/2011/11/10/142209900/video-why-gold

hi! more input is welcome, but meanwhile, get started on this section of the FAQ*

Why was gold always considered so valuable?

*see the link on the sidebar or the wiki tab