People value units of money because of their expected purchasing power; money will allow people to receive real goods and services in the future, and hence people are willing to give up real goods and services now in order to attain cash balances. Thus the expected future purchasing power of money explains its current purchasing power.
From a theoretical perspective, this is been the clearest elucidation to me. The reason the value of bitcoins is going up is because people who buy bitcoins expect that others will pay a higher price for it in the future.
Like I tell people: people who buy and sell bitcoins don't care that you don't want to participate. They just want anyone else who will participate.