Mainly just emotionally growing up along with developing a general self-awareness that being a sore loser makes you look pathetic.

I used to get way too mad playing online fighting games like Street Fighter 4, but thankfully I got over it by realizing a few things:

1. I wouldn't dream of reacting that way to losing in person, because I know that it's an embarrassing way for a grown ass man or woman to react to losing a game. It's fine to play games competitively and anyone who does knows that it can be frustrating to lose. But you only end up losing twice if you make yourself look like a petty little shithead. I'm guessing you're playing online games, but just ask yourself if you would act the same way in an offline setting--most people wouldn't, so you probably shouldn't.

2. You're probably losing because you're doing a bad job, and lashing out at yourself, your opponent or your teammates robs you of any chance to learn or improve.

I'm mostly a fan of fighting games and solo RTS, but I've played my fair share of CS, Apex and other team-based games. Solo games are simple. If you're playing a fighting game and you're getting hit by stupid annoying shit, it's your fault for hitting buttons at the wrong time, for being on your back foot or not knowing how to block, plain and simple. If you're playing an RTS and you get cheesed with a canon rush or some gimmicky shit, that's also something that you need to be able to deal with.

If you're playing a team game you might be tempted to flame your teammates for losing a match, but you either need to play with people who have the same level of skill and dedication as you, become good enough alone to carry a team of randoms, or simply care less about winning and focus on your own individual performance. Don't waste your time and energy being a dick to your teammates whether you think they're bad or not, if you're not playing on an established team then you don't really have any business taking that game super seriously and it reflects more badly on you than being good at a game. If you're playing with random people, expect random results.

1. If you care about actually being good, then long-time improvement is much more important than short-term wins and losses. Nobody can win every time and just because you lose once doesn't mean you're bad. Conversely, it's just as easy to get a fraudulent win off of a fluke or bad play from your opponent(s). So, if you really want to be good at a game you have to have other, better metrics for how well you're actually playing.

If I'm playing Street Fighter for example, instead of focusing on individual wins and loses in a single play session, I'd much rather focus on improving some aspect of my gameplay that I know is weak. Maybe I don't hit my anti-airs enough, so I want to focus on punishing jumps as much as possible. Maybe I'm not taking advantage of opening my opponent up enough, so I want to focus on consistently hitting bigger combos. I find that setting a goal for myself other than "just win as much as possible", helps me to keep my head in the right place while actually improving as a player.

So, TLDR: If you wouldn't act like a little bitch about losing in person, then you shouldn't do it at home either because it's a bad look and you're making yourself a bigger loser for no reason. If you lash out at your opponent or teammates for losing instead of reflecting on your own performance, then you're just doing yourself a disservice when it comes to getting better. And finally, winning and losing doesn't mean shit compared to actually making sure you're playing well, so change your metrics for success to things that you know are more meaningful.