What are the factors contributing to the value fluctuation of a currency ?

Answer 1

The most important factors necessary for a strong currency are low inflation, an open economy, reasonable regulations and low tax and interest rates.

So any change in these factors would influence the value of a currency.

Ex. the dollar is currently going down with respect to other major currencies, because the FED has started to lower interest rates and our HUGE deficits.

Answer 2

Absent government and central bank interference (which do occur in the real world), that would be inflation, interest rates, trade flows, capital flows, and expectations.