i agree with the main question being asked, but not the reasoning. I feel it's mostly due to the fact that playing with friends promotes less toxicity and more cohesion overall. Since this is a team game riot rewards team players. The removal of solo que during one of the recent seasons should give this more bearing.
Trying to force people to talk to each other and reach out though is pretty disrespectful of the developers. For example my state has laws that specifically prohibits the idea of social interaction with strangers online. (I know, I know. It's the dumbest thing I've ever heard). It's considered stalking and harassment.
http://victimsofcrime.org/our-programs/stalking-resource-center/stalking-laws/criminal-stalking-laws-by-state/tennessee#315
http://www.lexisnexis.com/hottopics/tncode/
so... idk. some people have stated that the reason why overwatch is less toxic when compared to league or dota is that it's less competitive and others have said that it's because you pay for the game. personally i think it's because they're not forcing you to play with other players for hours on end. you can drop in and drop out for the most part without penalties and 1 person can still do really well on their own since the game doesnt have a leveling or currency system in game. you start with a kit and that kit is the same from beginning to finish.