Why is Tony always angry at Steve, specifically why was he so mad at Steve in Endgame?

Because Tony's an ordinary human being.

And I really do mean that.

While Tony is surrounded by gods, super soldiers, and other people who have undergone "enhancement," he considers himself to be "just a man in a can."

Tony is not always logical, just like the majority of people. He's also not the most courteous, tactful, or nice person.

In reality, he exhibits traits like egotism, resentment, emotion, and fear. His motivations are fear and regret during the second and third Avengers movies as well as Captain America: Civil War.

In Endgame, Tony is upset with Steve because he believes that what happened in Infinity War validated his judgement and actions in Age of Ultron. And that in contrast, despite his good intentions, Steve's actions essentially ensured that the Avengers would lose Infinity War and that half of the universe's life would perish.

Of course he's wrong, but that's how he feels.

And it is currently both Tony's and the entire human race's problem:

Facts are irrelevant.

Feelings solely matter.

Tony makes decisions based on his emotions. He acts on his feelings, not always what makes sense. or morally correct.

Since Loki attempted to take over the planet in the first Avengers movie, Tony has been terrified—and by terrified, I mean truly terrified—of an alien invasion. When Wanda Maximoff entered his thoughts in the first moments of Age of Ultron, he saw the entire team as defeated and dead, which only made his terror worse.

Tony promised himself he wouldn't allow it to happen

and in his conceit and terror, he thought he alone was wise enough, kind enough, and resolute enough to defend the Earth.

Additionally, he would not accept any other person's complaints. Tony believed that the goals outweighed the means. He aggressively argued in favour of a "suit of armour around the planet," and when his team members objected to such a possibly hazardous, authoritarian, fear-based idea, he derided and belittled them. Bruce Banner was even coerced by Tony into helping him construct Ultron and then Vision, against his misgivings. Thor and Steve were both annoyed by Tony's conviction that he had made the right decision, even after Ultron malfunctioned and destroyed J.A.R.V.I.S. Tony also believed that his heart was in the right place.

Cap and the others are reluctant to restrict people's freedoms and/or tramp all over their rights in an effort to stop such a threat from materializing because they think the Avengers are powerful enough to handle any extraterrestrial menace. However, Tony has no qualms about taking any action he deems necessary in the service of his own arbitrary concept of safety because he is paralyzed with fear and suffers from a massive savior complex.

Tony's greatest fears are realized as Thanos and the Black Order invade Earth, take the Infinity Stones, and cause the metaphorical "Snap." Arriving is a formidable extraterrestrial menace. The Avengers have been defeated. The civilian population of Earth is halved off. Tony has been dreading this particular situation for years.

Thus, it makes sense why he is angry. He is actually experiencing his darkest fear. He is now confronted by the person he fears the most: Thanos. Half of Earth's population has perished as a result of Avengers defeat. Tony believes that these incidents demonstrate his point that safety comes before rights. And he directs his wrath at Steve because it was Steve who vehemently opposed Tony's efforts to develop Ultron and Vision as well as his idea of a "suit of armor encircling the Earth."

So Tony becomes upset with Steve for debating him and disagreeing him in Age of Ultron, behaving like the vengeful, petty, egotistical, and unreasonable human being he is. Instead of offering suggestions, Tony wallows in self-pity, berating Cap for not paying attention to him, blaming Cap for his absence (which was brought on by the events of the Civil War), and essentially saying, "I told you this would happen."

Tony's confirmation bias would read Thanos's triumph as indisputable evidence that Tony was correct to demand the construction of a suit of armour over the globe, and Tony would place all responsibility for the failure to erect said armour at Steve's door.

He believes he could have defeated Thanos with a plan that would have worked. And he holds Steve accountable for derailing his plans and ensuring the Avengers' demise.

He is upset because of this in Endgame. In addition to being scared, humiliated, and in mourning, he is also furious that no one paid attention to him. Particularly Steve, whose moral influence might have influenced the other team members and given legitimacy to Tony's ideas. Which, according to Tony, might have stopped the "Snap" from occurring.

Keeping all there differences aside Tony has huge amount of respect for Steve Rogers and that is portrayed throughout the Infinity Saga. In turn Steve trusts Tony to death, and earning the trust of Captain America is not an easy task. Both of them are good work friends and surprisingly Tony has no problem working under Cap more so over he acknowledges Cap's leading skills. We can see in Civil War, that Tony tries his best in not letting the team split (until he got to know about his parents murder), which showed that he truly cared.