How do exchanges decide on cryptocurrency trading pairs? Cryptocurrency trading pairs for example ltc/btc, doge/btc should be paired using some criteria. I'd like to comprehend on what basis they are paired.
Exchanges generally choose a few base currencies and build trading pairs on top of them. It has multiple reasons:


The exchange needs to find a base currency where people can understand the value of X from X/base. BTC/USD is $1350 that gives you a clear idea of how much BTC is worth in USD. That is different from LTC/DRK = 0.98. What did that tell you? Not much. You are in need of a base currency you are familiar with. Most people have a good feeling for the value of USD. Therefore, it is a good denominator.
The exchange needs to find a base currency which people are most likely to deposit/withdraw for trading. After you are done with trading numerous altcoins with each other, you want to exit the trading platform in a currency that has a meaning to you. To make that easy, base pairs are usually BTC.
This is probably the most important, if an exchange has BTC LTC USD, they will most likely not create BTC/LTC pair because that will decrease the liquidity of LTC/USD pair. They will most likely only offer BTC/USD and LTC/USD. On top of that, the extra pair will have maintenance cost to the exchange.