I have noticed today that Bitcoin (BTC) and Ethereum (ETH) charts look almost the same.
Why are they so correlated?
All the crypto currencies are currently riding mostly on FOMO, as there is no other value behind them (FOMO = Fear Of Missing Out). People see that they are rising, so they are afraid of missing the big thing, so more and more get into them (which makes them rise even more, etc.)
This mood is pretty much identical for nearly all of them, so they all move in sync.
There is a lot of discussion about the longterm usefulness and value of a crypto currency, but if interest disappears tomorrow, they are all only random bit strings on some harddisk, and as such not worth a penny. On the other hand, interest might stay or increase even more, so they might rise to much higher prices. It’s all in peoples minds.
There are several reasons for this strong correlation:
That being said, the prices are definitely not strictly coupled long term. See the golden line at https://coinmarketcap.com/currencies/ethereum/ which would remain flat with perfect correlation.