Why does other cryptos value goes with bitcoin in terms of up and down?

Bitcoin is the trading pair of practically every other cryptocurrency on the exchanges. A lot of smaller cap cryptocurrencies do not have other trading pairs. What this means is that all the money that goes into those cryptocurrencies and away from them has to go through Bitcoin. At the start of a bull run, all the new money goes into Bitcoin first and from there to the other cryptos, Conversely, when a bear market starts, investors pull their money away from altcoins (= other cryptos) into BTC and from there back to dollars/euros/yen/etc. Bitcoin has the largest market capitalization (market cap = units in circulation x price per unit) of all cryptocurrencies and it is the least susceptible to random fluctuations. That also explains its lower volatility.

More of the altcoins are bought by exchanging bitcoin. Let's assume bitcoin as a piece of bread costing 12$ and you could buy any groceries in the market by exchanging bread. Suddenly if bread price went to 6$ then the amount of groceries you get would alter(decrease). Similar happens with the uptrend.
** Most importantly decrease in btc price might not make alts costlier the reverse also may happen because many people start dumping their btc to some alts and the supply of alts increases as a result it may become cheaper, similar happens when btc goes up.

The reserve currency for crypto is bitcoin at the moment.  Most recognizable, accepted, traded therefore most valued by the masses.  It as a whole realizes less volatility than the smaller coins or alt coins on the platforms so they are more of a safer haven when volatility in crypto is expected.

because bitcoin is a mother currency and all other coins are dealt with btc while trading, as someone wants to buy a coin they will buy btc and then that coin, and also when an ico comes the company takes your btc and gives you token and if the value of btc falls , then the amount of your investment company have to run its business also falls and with that the altcoin value also falls.
Bitcoin is the founder currency and holds the majority shares in the crypto world. Market dominance of bitcoin is around 32-35% of total crypto currency volume.
There are about 1900 plus crypto currencies and no other coin has such a huge dominance. Therefore, all the crypto market follows the Bitcoin.
Some of the coins are getting trust of the investors are these coins. With increases dominance of other coins in future we may expect that crypto market will not be dependent on one single coin but for this we have to wait for a long time.

BTC is the reserve currency for cryptos as of right now, with a few others trailing in a much more minor effect due to BTC visibility as the generic term for many people in the public right now that don't follow or follow little if at all.  People that are more hardcore followers have their own opinions in ways of hedging themselves in the crypto world but until BTC is no longer the most visible and until more of the general public looks to understand crypto that is the way it's going to be.  Has little to do with what actual BTC does but acts more like an exchange for all things crypto right now as far as market prices go.

The cause is mostly because of people's perception or as most them like to call it: FUD (Fear Uncertainty Doubt)

Right now, over half of the people who went into crypto business doesn't fully understand how it works. To top it all of, the majority of them "only" know about Bitcoin.

So when something happens to Bitcoin (like when the government or media spreads false rumor about it, or when an exchange gets hack or something), It is expected that other alt coins will follow because it will trickle-down to the lower alt coins
From bitcoin standpoint, there is only one real crypto currency bitcoin, rest are all shitcoins.

We can talk about Technical analysis and forecasting of all the shitcoins but at the end of the day they move with btc. the whole world of cryptocurrency is built around bitcoin
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Bitcoin is the major cryptocurrency and that is why most cryptocurrencies are exchanged with

it,and since most cryptocurrencies are trades with it then it will be alright expected that whatever

happens to bitcoin will also affect the other cryptocurrency so that is why whenever the

bitcoin price goes up then the value of other cryptocurrencies goes up too and when bitcoin

price goes down then the value of ofther cryptocurrencies goes down too...
Point 1: Most of the currencies are bought as BTC pairs so BTC goes up they go up, BTC goes down they go down.

Point 2: FUD, fear uncertainty doubt, as no one is sure where a downtrend is going to stop people start selling to buy again at lower price which effects the price of a coin more than the drop of the BTC percent.
Bitcoin is the mother currency of other cryptocurrency at first only bitcoin was the coin in this world at time of 2009 then after other cryptocurrency started coming in the platform under bitcoin so the coins are on bitcoin coding so this is the main reason after bitcoin price and other altcoin price

Because they are based off of the base value of a bitcoin which is a satoshi. They get their value form how many satoshi they are worth. So when bitcoin goes up so do they.