When not to castle

Castling usually brings your king to safety, but sometimes it can bring your king to checkmate.  This is what happened in this game with my opponent, I was attacking the g7 square when my opponent castled, which led me to checkmate when I captured g7.  BTW I am still getting used to this whole blogging thing so I hope these blogs are not to boring.  Any feedback would be appreciated.  Disclaimer, I am not a chess expert, or master.  Just a rank amateur who is trying to share some observations and learn from the community at large.