Spike is always looking for ways to be helpful. So, when an opportunity to babysit Flurry Heart so Princess Cadence and Prince Shining Armor can have a weekend together comes up, Spike is quick to take on the opportunity of responsibility.
Spike has bravely helped to save the Crystal Empire from King Sombra and melted a lethal ice cloud during The Equestrian Games. So watching over a baby Alicorn should be easy for the "Brave and Glorious" dragon, right?