Female dragons lay the eggs, and the males take over from there. So when Spike finds a dead male dragon with an egg, instincts kick in. There's no way he's going to let the hatchling die. However, the unhatched egg brings some questions into light. Who is Spike? How was Twilight able to hatch him? Where did Spike come from and where is his father? The answers to these question, along with raising a baby dragon, may cost Spike everything.

[img]http://fc09.deviantart.net/fs70/f/2013/242/3/9/mlp__a_dragon_s_nest_by_yaoi0yuri0rules-d6kcdq1.png[/img]
(Original dragon creator by wyndbain. DeviantART of wyndbain)