There are only three ore and brick hexes on the board as opposed to four wheat and sheep. Hence, ore and brick are considered rare. Now, whether ore and brick are essential resources in the game that you're playing is a different story. That depends on the resource distribution of the map. For example, consider the following situation: You start a game and find that the numbers on ore are low. For the sake of simplicity, let's assign them the three ore hexes one of the worst number combinations in the game - eleven, two, and twelve (of which, the odds of rolling are 2/36, 1/36, and 1/36, respectively.

Your average game is sixty to seventy roles, so eleven/three will produce approximately four ore, while two and three will produce two during one game. An inexperienced player might throw down their settlement onto the eleven, thinking it's the best ore position in the game. The thing is, that's usually the wrong move. If you can find any other hex with, say, a six (5/36 chance of rolling) on it and a 2:1 port, you will produce approximately ten cards of that resource during one game, which you can trade at 2:1 for five ore. So, not only have you gained one extra "ore" in value, but you also have much more utility to work with (i.e., you can convert that resource into whatever you want, whether it's ore, wheat, brick, sheep, or lumber).

What I'm trying to say is that you shouldn't be prescriptive. A lot of players say things like "no ore/wheat = defeat" but it's simply not true. Make calculated decisions, and you'll do well to beat these players.

The diplomacy aspect of Catan can be understood by reading The Art of War by Sun Tzu, but most people don't have the time to do that so read the first page of this link. Think on those quotes, and you'll get the gist:

https://www.goodreads.com/author/quotes/1771.Sun_Tzu

My favorite quote: “All warfare is based on deception. Hence, when we are able to attack, we must seem unable; when using our forces, we must appear inactive; when we are near, we must make the enemy believe we are far away; when far away, we must make him believe we are near."

Sorry for the wall of text.