Katsuki Bakugo makes quite a name for himself in My Hero Academia as the notably angry and ironically explosive rival to Izuku Midoriya. Compared to Midoriya, Bakugo is always angry and quick to express it, especially in violent ways, whether yelling or letting off some small explosions to make his point. However, neither the anime nor the manga gives a real reason as to why Bakugo is so explosive (pun intended).
Fortunately, there are enough fan theories out there to supply some pretty good answers to this question. While none of these are canon, they hit the nail on the head and posit some very believable explanations.
One of the most scientific theories out there explains Bakugo's anger quite well. User Betelhem Judkins suggests on Quora that his fiery personality is due to the nitroglycerin in his sweat. The nitroglycerin is what gives him his explosive powers, as he's able to ignite it. However, nitroglycerin causes the heart to slow down, so the theory suggests that if Bakugo doesn't yell, scream or somehow maintain his high energy levels, he may literally pass out because his heart is slowed down all the time. While there's no way to verify whether this is true or not, it makes sense logically. The nitroglycerin would force him to maintain his exuberant behavior for his own safety.
Another theory takes a different but equally valid approach. User Annie Cotter on Quora suggests that he's so angry all the time because he has a serious superiority complex. She explains it well: Bakugo spent his entire childhood being praised for having such a powerful Quirk, with people showering him with compliments for most of his life. This caused a clear distinction for him between people who had Quirks and people like Midoriya who did not; having a Quirk makes you better. Spending years being told he's amazing because of his Quirk, Bakugo began to need that praise.
However, when he shows up at UA and realizes that not only do many people have powerful Quirks, but also that Midoriya, who Bakugo had previously bullied for not having a Quirk, suddenly has a powerful one. Bakugo therefore developed a superiority complex. His ego took a major hit when he arrived at UA to find himself surrounded by other powerful Quirks, and this fuelled his anger. Unable to deal with these new emotions and feeling like he's less than the best, Bakugo expresses it the only way he knows how -- through anger and lashing out.
One other theory suggests similar reasons for Bakugo's anger but includes a few more details. User John Smith on Quora agrees with the previous theory that Bakugo has a superiority complex due to his constant praise as a child. However, they also suggest that Bakugo's parents had a lot to do with the way Bakugo is. The few times we've seen Bakugo's mother, the resemblance is uncanny. She exhibits the same shows of anger and violence that Bakugo does, even hitting Bakugo at times.
Although it's played off as something funny, this would explain where Bakugo gets it from. His mother has even made a point of calling him out on his weakness, which inevitably forces Bakugo to work even harder. Additionally, Bakugo now associates weakness with failure, which certainly doesn't help his already angry disposition.
These theories help build a better picture as to why Bakugo is the way he is. They help define his character a little more and provide important and contextual details from his backstory that highlight things that would explain his constantly angry demeanor. Ultimately, Bakugo being angry all the time doesn't make him a bad guy -- it just makes him a pain to deal with. However, as he progresses as a Pro Hero, Bakugo is starting to unlearn some of his less-than-favorable behaviors in order to achieve his goal of becoming the number one Pro Hero in the world. His anger only motivates him to be better than he once was, which can only work out for the best for all involved.