Cuz you want free elo :D

Why pick Amumu for jungle? because he has OP ganks with his Q and later with his ultimate too, and because he is a powerfull tank in teamfights, where he can stun whole team and take down the healthy tanks with his W

With this combination you will get defensive early and late game + strong damage output early game.

Take defensive masteries and bonus movement speed and buff duration. remember to improve your summoner spells.

You need expensive items, therefore you need gold+ items as fast as possible. Then focus defense items, which at the same time improves your offens. Therefore take Sunfire cape and Abyssal Scepter. Then focus on defensive items. Take Randuin's Omen and Force of Nature to make sure your enemies do not escape. You can slow with the active of Randuin's Omen and catch them up with the bonus movement speed for Force of Nature. At last pick Shurelya's Reverie for ekstra movement speed and to improve your teams engage and escape posibilities in team fights

Pick Flash for engage/escape and Smite for jungling and to steal/protect dragon and baron