The short scenario for this picture is this: Sometime after Rainbow Dash left the Wonderbolts, she decided to start her own flying team called the Rainbooms. With the help of her friends it was very successful and a celebration was just dying to happen. Of course Rarity (a major contributor since she made the costumes) insisted it be a formal affair at a fancy restaurant with dresses and frou pillows, all that stuff. Not Rainbow’s cup of tea, but all of her friends seem to oddly want Rarity’s idea. So Dash reluctantly goes along with it.
 
But Rarity had her own secret side reason for the more fancy occasion. Rainbow Dash had no idea he would show up. And while Dumbbell’s gesture with flowers is genuine, he can’t argue that watching Dash flounder under his all of a sudden gentlemanly approach isn’t the most amusing thing ever.
 
Also Dumbbell helped her design the costume, then they gave the idea to Rarity to fine tune and revise and work her dress maker magic. During that time she got to know Dumbbell more, which is why she is instigating this interaction.