Barney and Robin were something of a surprise for many fans of How I Met Your Mother - Barney seemed like the least likely person in New York to get married, and Robin made it clear that she didn't want a serious relationship, let alone with someone who took such pride in his 'plays'. But over time, it became clear that these two actually had a lot in common.
Later in the series, entire seasons were devoted to the run-up to their big day, with flash-forwards and eventually, multiple episodes that focused on their wedding, but it worked - and fans loved seeing them together. In the end, though, after all their on-again, off-agains, and their marriage, they end up in the future timeline divorced... but always friends.
Toxic: Game Playing
Barney is known for running plays and playing games with women, and even though he loves Robin, he can't seem to break this habit. When he proposes, he actually runs an incredibly long and convoluted play, lying to her and manipulating her into realizing that she loves him, too. It may end with a romantic snowy moment, and a big 'yes', but this is also painfully toxic, as their marriage starts with an engagement based on playing mind games.
Perfect: They Both Play The Same Games
On the flip side, Barney isn't the only one to play games. When Robin realizes that she has feelings for him again, she runs a series of her own 'plays', including dres as Lara Croft to play laser tag, and eventually showing up at his door in lingerie to seduce him. Perhaps this is something that they have in common, that they actually both appreciate this kind of thought and intentional manipulation, and it means that they suit each other.
Toxic: Constant Fighting
When Robin and Barney first get together, they try and play the perfect couple for a long time... but it soon comes out that they are actually fighting constantly. This becomes something of a hallmark for them, too.
They fight through a ski trip, they fight through their relationships, they fight about wedding plans (and a ring bear), and when Robin thinks she might be pregnant, she even gets physically violent, punching him in the face and knocking him out.
Perfect: Amazing Chemistry
There is absolutely no doubt that Barney and Robin have some of the best chemistry of any couple on the show. Their scenes dancing together at a wedding are phenomenally charged, and it's clear that, throughout the seasons, they just can't keep their hands off each other. It's not everything, in a relationship, but having this kind of chemistry and ion certainly helps.
Toxic: Bring Out The Worst In Each Other
At the end of one of their times together, this is actually the reason that the two break up. Despite being 'happy', their relationship leads to both Barney and Robin no longer treating themselves well - it's not just that they 'let themselves go', but that they no longer take care of or pride in themselves. Robin also becomes mean and snappy, while Barney becomes stupid and oblivious to social cues. This is absolutely not what a healthy relationship should do - ideally, it brings out the best in the people in it!
Perfect: Mutual Interests
While Robin might not be quite as into laser tag as Barney is, they actually have a lot in common. Both love good whiskey, cigars, and suits (to different degrees, of course), both are blunt and independent (and ambitious), both have the same sense of humor. And along with the rest of the friend group, they enjoy doing the same things - because being friends is a vital part of being in a great relationship.
Toxic: Commitment Phobic
Both Barney and Robin have some serious issues with commitment. Barney is so terrified of a relationship that he sleepwalks to the window to sneak out in the early nights of their time together, and while Robin isn't that bad, she's also far from ready to settle down.
In order to even get them to initially decide that they are in a relationship, Lily has to lock them in a room and literally make them talk it out.
Perfect: Same Goals In Life
A huge factor in how long a relationship will last is if people have the same goals. One of the reasons that Ted and Robin didn't make it (until that controversial finale, anyway) was that Ted wanted to settle down, have kids, and live a quiet life... but Robin wanted to travel the world, child-free. When it comes to Barney, though, his and Robin's goals lined up. He was also ambitious and had no interest in children, and was happy to follow Robin as she pursued her dreams.
Toxic: Cheating
When Barney was with Nora, and Robin was with Kevin, the two ended up cheating together - which is rarely a good sign for a couple. At least, they both talked about being honest with their partners, and it was Barney who actually spoke up, itted what happened, and ended things. However, Robin didn't, deciding that Kevin was the one to stay with. No matter what the fallout, though, the mutual decision to cheat is not a good one.
Perfect: They Grew Together
They may have a lot of things in common on a more superficial level, but these two also seemed to really understand each other deep down - and they got there over time, helping each other grow. They were able (with some breaks) to find a way to be exes and friends at the same time, and their relationships helped them figure out what they wanted, where they were going, and how to be the best versions of themselves.