I've read on several different occasions that Roleplaying games are like having a play gunfight as kid and then someone says "I shot you" and the friend calls out "no you didn't"
We are playing the same games of make believe. Cops and robbers, space ships and aliens, monkey bars and lava monsters. The difference is that we use a set of rules to arbitrate when someone says "I shot you"
Various rule systems exist to mediate these acts. Probabilities of success or failure weighted and recorded on a vital piece of paper.
I like different genres and the different rule systems that accompany them. Rules can serve to help convey a feel or concept important to the game setting as well as reward or punish certain behavior.
For example in early Dungeons and Dragons Armor Class is an abstracted concept that determined if you were hurt or not. It didn't matter if you were actually hit or not. As a result Fighters were rewarded for wearing the best armor they could and there was very little incentive to be a swashbuckler type.
The key thing is that the rules provide a common understanding between all the players. Rules can be cut out if they feel too clunky or obtrusive to the game, house rules can be added to give an individual flair.
Often times at a gaming table I have seen (and am guilty of this in my youth as well) a GM change the results of dice because they didn't like the results. A result that destroys the villain prematurely, or would wipe out a player. Heck, I've even seen books dedicated to altering the results of the die rolls for the purpose of keeping the game going smoothly.
Sometimes the climactic finish all comes down to a single roll, a great nail biting moment.
But if you are going to change what the dice say, why bother rolling them anyway? If a situation is so important that the GM is not willing to accept a roll, then they just shouldn't. Narrate through the moment. If the big bad can get killed in a single roll, and it happens then revel with the players in their success. If a player gets a bad roll and a character dies -well while it is the end of that character, it can be a teaching point for the player, what did they do to get their character in that situation in the first place? Even if they did everything right, sometimes adventurers fail- but it makes the success of the next hero they play that much more rewarding.
As a GM if you use random tables stick with the randomness and work you imagination. The dice have provided the result, you provide the context. After all, it should never be you vs. the players, it is you with the players, you have the responsibility of setting the scene. If some tables give you inspiration you can just pick from them as well, the RPG police aren't going to knock down your door and MAKE your roll, they stopped doing that years ago.
As a player if a character is built as a min/max to "always win" munchkin then they are trying to play with only the illusion of failure. While sure winning is fun, everyone likes to win. It is like playing a video game with cheat codes. You don't get any better at playing the game, and the victory is hollow. The dragon's treasure horde is meaningless, and only has the value of a participation trophy.
If something isn't working look at the rules, and agree on changes. When the dice are rolled though let them lie where they may.