[conroe64]

In order to exchange them someone needs to buy them first. So, depending on how much they are used for trade, the amount held transitively for this purpose will vary.

Since there is a fixed number, the more popular they become as a trading platform, the higher the price will go, since if many people, P, need to trade at the same time, and they need an average of $X worth at any given time, and there are B bitcoins available, then $X * P / B is the minimum price a bitcoin can be.

So, if the amount used for trade becomes somewhat stable and/or continues to grow, it means there is some real base dollar value for a bitcoin in order to support this. Given this base value, along with its other features of easily being kept safe and transferred across national borders, it then makes them attractive to store wealth, which increases the value much more. And then, given the group of people willing to store wealth in bitcoin, it makes people even more confident in it as a store of wealth. And so on and so on...