Hmm this seems like a good way for how Fluttershy and Mac started. Fluttershy got hammered on some spiked (by spike) punch. Before she new it she was a bit sloshed. The party ends and she is a bit worse for where. AJ asks her brother to help her home, knowing he would be a total gentle man. He does as asked and as he helps her to her door. Fluttershy blurts out what she was always to shy to say. "I THINK YOUR HOT!" This causes Mac to loose balance while holding her up. They both fall to the ground with Fluttershy landing on him. They look each other in the eyes and Fluttershy makes one more bold move she never would have done. She kisses him. This makes her come back to her senses as she quickly gets and retreats into her house. Leaving Mac a far deeper shade of red then he is use to being and even more confused.
Thus their shy and awkward relationship starts. At least, that's how it could happen. If caused by a Pinkie Pie Party.