They need to be pretty.  
They were designed specifically for that purpose, it’s what their species has evolved to be: pretty.  
When their gods created the world in which they dwell, they made it specifically so the dominant species of that realm would be sure to be one thing: pretty.  
In that world, to be pretty is to be superior: to be higher up in the food chain!  
And as such, as their planet’s ultimate species, they must be pretty, for their prettyness is the very symbol of their position in the top of the food chain.

They don’t stand a chance of saving Equestria unless they’re pretty. It’s simple narrative logic.

@darklord99100  

 
That reminds me of Hot Tub Time Machine, where the one guy was yelling on the phone at his under-aged future wife.

“You hear I’m pretty?! Can’t you make up your own mind?!”

But, unfortunately, they were never meant to be.

Not ENTIRELY pretty, though…

“…we need to be…”  
Discreet?  
Sure?

Do you need an adult, Dashie?

@darklord99100  
i mean flirting

i guess Twi couldn’t prevent herself from talking to her marefriend in the past