Recently I have been told by a lot of people (both on the forums and in person) that my campaign's are extremely challenging.

To deflect this, I usually point to the DMG, and how I typically follow the guidelines in it pretty much too the letter, with each adventuring day using up roughly ~80% of the party's resources. If anything I actually softball it a little, as I don't use the part where it says "5% of all encounters should be overwhelming difficult and dealt with by running" or that most encounters should include a few scary moments.

But still, the feeling of challenge is there.

So are the guidelines in the DMG too hard?

If so, what is the right level of challenge? And how do you run an easier game without breaking the system or the setting?

If not, how do you get people more used to the "expected" level of difficulty?

Now, some speculation about what I might be doing wrong:

1: My players do not memorize the monster manual, and I often use customized or reskinned monsters. If my players don't have any information gathering or knowledge abilities, they normally learn their opponent's abilities by doing over the course of the encounter.

2: I often place "optional" monsters in dungeons which are not required to complete the main plot and are in addition to the standard CR budget, but provide additional XP and / or treasure. To most players, they may not seem optional at all.

3: I might just play monsters too smart. I typically allow them to use tactics, prepare for the fight, and make use of the terrain. For example, I remember back in the college gaming club being sad that the other DM always had more players, and one of the guys who was in both games said people came to D&D for big dumb fun, and compared a fight where he had an ancient half shadow half fiendish red dragon with class levels charge in and brawl with the PCs, while I had a standard young adult green dragon use cover, camouflage, deadfall traps, and low level buff spells, and both were able to be of a similar level of difficulty to the party.

Spoiler: Details about my specific game:

We just completed a campaign where we played every other week for almost two years.

So, basically, I have four players:
One of them bitches about basically every encounter and finds someone or something to blame anytime his character fails at something.
One of them is normally fine, but occasionally, usually when encountering a monster that he can't just run up to and trade full attacks with, or when he is wrong about a rule, he explodes, calls people (usually me) names, screams, and threatens to quit the game.
The other two were pretty calm and drama free, but during the last half-dozen sessions or so they started exhibiting the same behavior as the first two, and I don't know it is the other players rubbing off on them, my game driving them to it, or some combination of the above.

Some data about my game(s):

I typically run about six encounters a session.

The players complain that they are forced to spend too much money on consumables, but are still significantly above WBL the entire game.

About once every five sessions they have a close fight where several of the players are down and they are seriously considering retreating to avoid a TPK, but pull through and win in the end.

About once every ten sessions the players will have an encounter where they are unable to achieve their goals the first time. They decide to fall back and regroup / resupply / research / ask for help, the enemy gets away and has to be tracked down, or the enemy incapacitates them on their first encounter.

About once every twenty sessions the party suffers a serious setback; the fail to stop the villain, they are forced to abandon the mission, one of the players dies (and resurrections isn't recoverable), they get their allies killed, or they make a mistake and choose the wrong side.

About once every fifty sessions the group actually suffers a TPK and either starts over or has to resort to a deus ex machina.