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why ltc, usually coins go up and down, why you believe that ltc will keep going up ?

The price is litecoin is very undervalued, it will go up eventually.
Fees to send Litecoin is very less which makes it useful for day-to-day transactions.
Faster speed,
Atomic swap,
are other reasons

While BTC will be used as asset (There will be less transactions of BTC, will not be used for daily transactions).
LTC will be used for daily transactions.

I too see LTC only seriously going up in value once exchanges offer more LTC pairs for trading instead of just LTC vs fiat or Tether. So people either invest to hodl LTC or are then forced to convert into BTC as one has to have BTC in order to trade most of the other altcoins. With all the recent surge of interest & new investors coming into the markets, the number of BTC transactions rocketed. With LTC's faster transaction times, other things like arbitrage will also be more feasible... OTOH BTC too will have faster transactions once Lightning comes online, so that won't be a USP for LTC anymore.