For example today I was vs. Voli top with Rek. I rushed Hydra, he rushed Randuins, should I have rushed Randuins? Why?
I've noticed lately that i'm a little confused as to which I should do and which circumstance I should do it, if someone could give me a good explanation that would be super helpful :)
The general rule is damage first, because it doesn't matter how long you live if you can't kill anything in that time.
If your opponent is ahead in damage, get defense. You wont be killing them on your own, so at least try not to die either.
As tanks against physical damage champions, I'd recommend rushing Sunfire Cape because of the defensive stats and that the aura contributes to your damage in fights. If greatly increases your survivability while helping you push the lane and gives you damage in fights.
Ravenous Hydra isn't really a great item on Volibear anyways. If you want a damage item, I would recommend one of the following:
Attack Speed lets you get your W up more quickly and can let your ultimate's on-hit damage occur more often.
is usually what I build first on that alligator, for the wave clear. I usually do not finish it to later though. Really you can build whatever you need vs your lane and the enemy team.
Volibear is a champion that excels damage wise not due to damage items, but due to his stats. You need to build him tank to be effective in fights, randuins, spirit visage, banshee's veil, mercury treads, thornmail, warmogs
That being said, Rek'Sai pretty much crushes everyone in a straight 1v1 right now - so you should have been focusing on building to fight with the items i mentioned above.
Build damage if you're performing well or your jungler is tank and support is tank Build tank if you are losing or your jungler is damage.
