Both Fluttershy and Big Macintosh are very soft spoken people but also passionate and hardworking.  
They knew each other well from being in the Pony Tones together however that’s not where their romantic relationship starts.  
Sweet Apple Acres was having a pest problem of fruit bats once again and Apple Jack asked Fluttershy to help.  
What Fluttershy didn’t know was that she’d be working with Big Mac since AJ had to deal with a friendship mission at the last minute.  
They worked most of the day in silence, relocating fruit bats to the Everfree forest until a Timberwolf came out of nowhere and tried to swipe at Fluttershy.  
Before she could even react, Big Mac pushed her out of the way and took a big scratch to his ankle.  
Fluttershy used her stare to scare the timberwolf off and helped Big Mac back to Sweet Apple acres where she bandaged him up.
 
 
************  
The bright, yellow Pegasus carefully wrapped the stallions ankle occasionally taking glances at him.
 
F: Thank you.
 
BM: Hmm?
 
The stallion looked at her with sleepy eyes.
 
F: Th-thank you, for saving me back there.
 
BM: Of course.
 
F:…
 
BM: What’s wrong?
 
F: Oh, I’m sorry!
 
Fluttershy began to flush out of embarrassment which made Big Mac chuckle.
 
F: It’s just besides singing, I’ve never heard you say more than one word at a time.
 
Big mac smiled sweetly.
 
BM:Yup, and I’ve never heard you yell the way you did at that timberwolf. Being such a small gal, you sure are brave.
 
F: Thank you.
 
Her heart began to flutter.
 

 
From that moment on they became closer.  
I also have a head cannon that they are the loudest and most vocal with each other and helped one another to be more vocal with other people
 
I traced the background from the show.