Because of Fluttershy's crippling shyness she's enrolled in junior flight camp. She meets bullies but none of them matters when she sees Dash; at that moment Fluttershy is obsessed. Her love causes her to take action, action some would deem deranged.
Luna saw her own darkness mimicked in the colt. If she could have someone who understood and could be with her in the darkness. Luna wouldn't hesitate to take the chance to change her own fate; even if it meant raising a monster.