Every item in the build aims to complement the others, and to shore up one of Jarvan's weaknesses.

Regrowth Pendant is one of my favorite starting items for an off-tank, because it provides a tremendous amount of staying power and can build into a wide variety of solid defensive items later in the game. For this character, I build a Warmog's mid-late game.

Mercury's Treads and Aegis of the Legion combine with Demacian Standard to give Jarvan a pretty good amount of armor and magic resistance - a little over 100 each.

Manamune and Youmuu's both provide Jarvan with some much-needed damage, and also help to reduce or eliminate his problems with mana and some rather long cool downs. Dragon Strike spam is a must if you want to remain a threat.

Finally, for my sixth item I will usually pick up Atma's, but if the other team is very magic-dependent Wit's End is an excellent substitute.

The only parts of the build that I would consider unchangeable are Manamune, Youmuu's, and either Atma's or Wit's End. Feel free to drop the Treads for Ninja Tabi, or Aegis/Warmog's for Banshee's Veil or Randuin's.