As with every relationship, it doesn't matter if it's love or hate, the strongest the relation the more harm will come if it is deteriorated. Both belong to the "passion" category and that's what is strong, not the love or hate. It can be twisted from love to hate and from hate to love in a jiffy. That's why you usually cannot forgive someone you had positive feelings for after they did something towards you which you consider terrible. [For a sports related documentary, Once Brothers from ESPN seems like a very good example].
The Icardi-Perisic saga started as the main duo and they seemed to have a deep appreciation for each other. Which is exactly why this escalated a lot. One side blamed the other for the piss poor performances, as if they couldn't just agree that playing with Nagatomo, Santon, Ranocchia and Felipe Melo was their biggest problem instead of each other. They were never really friends but you could see they had chemistry.
Have you ever played football or any sport with friends and some random guys? The norm is to yell at your friends and those who you are closer to than the random guys, even if they make sillier errors. Sure, you're friends so you think each other won't care after the game is over, but at some point you may accidentally reach a boiling point and you'll stop wanting to play with your friends. You are always harsher and more sincere with people you feel closer to.
Cases that involve off the pitch shit like Wanda (I wonder what Crespo thinks now
) are outliers and obviously have nothing to do with anything discussed about team chemistry and players getting along.
Another example is Ronaldo. He wasn't really a close friend with anyone at Inter other than a select few who he hanged out with, mostly Italians or whatever Brazilians we had during that time (includes Vampeta :alone: ). Like Ventola and Vieri in particular. But everyone liked him, even the Argentineans loved the guy. His work rate was shit, he was injured for like 2 years, but you won't hear anyone question his personality or professionalism. And I don't think he even cared that much that Julio Cesar married his girl, except maybe for that derby goal against us and the irony is that he even wanted to return whilst Julio Cesar was around. This is just too rare even for football players who live in their own small world sometimes.
All in all, it's not bad to see players forming friendships. In fact, it's a very nice sign for a club that intends to keep both around. But let's not overdo it and say that it's better to have this than not, how this is a sign of great team atmosphere [Perisic and Brozovic were also close last season, why weren't we praising that?] or how it enables the team to play better or whatever else has been written or will be written about this.
It's a side effect of working with some people. Some people are easy to speak to, some others are very sociable, some others are just loners that don't care about other people's feelings or even their own, some others are ambitious and/or pretentious, some are opportunists, some others are just naive and some others are just around for the ride. I've said it repeatedly. Professional football is not far different from a camp for people in their 20s. With their own money, own places and very few restrictions. And then you have the veterans. If they dictate the locker room and they're still good enough, you can see wonderful things (Mourinho's Inter, Milan in the mid 2000s with their CL runs etc), if it's just a couple of them amongst boys, they'll just be pissed off and won't perform well. Especially if they are brought from somewhere else. This is why squad building is not an easy task. You can easily say I want these 11 players to start and these 10 players to complete the roster, but that's just what you see every Sunday. A football club is a daily workplace like everything else and most of the time these people are free to express whatever they want as they aren't in school with a teacher for 10 hours a day. They barely have 3 hours of sessions where a coach dictates things per day. The rest of the day, they're having group spa treatment, pedicures, play video games, goof around or just play football where talking to each other is obviously not frowned upon... So from a club's perspective, players being too friendly may also be disadvantageous at some points because they could be less focused on their work during the week.