Well first and foremost opposites attract. However as other people have said they are actually quite similar, both of them had very bad childhoods, with Blake in the White Fang and Yang losing both of her mothers and then having to bottle up the pain and take care of Ruby. As well as someone else said Blake hasn't been able to having anything near a normal childhood and teenage life, with her Banzai being the first normal teenage thing she's done. And who better to show her how to be a normal teenager then perhaps the most normal teenage girl in the show? (for the most part)

As well Yang is there for Blake whenever she needs her, when Blake was falling apart this season Yang was the one to make sure she didn't outright destroy herself and to bring her back and make her slow down and enjoy herself, especially dancing with Yang first out of all people (ahead of Sun).

As Yang also deals with her demons this season with whoever Raven is (I still bet her mother) and whatever she tells her Blake will be there to pick up the pieces like Yang did for her this season and make sure she doesn't destroy herself as well. Yang also obviously doesn't have any reservations against faunus and even feels sorry for them.

Blake as well when they were fighting the paladin was generally fearful when the paladin smacked her into the concrete support compared to the other members of team RWBY, probably too fearful...

I'd also say like the other great pairings that they bring out the best in each other. Yang opening up Blake to a normal teenage life and showing her compassion and friendship while Blake give Yang someone who understands her deep pain and someone who will stick by her and ground her, ground each other.

Also they are just adorable together! Art with Yang rubbing Blake's are also so cute how couldn't you support this ship? ^{This may or may not be my OTP if that was noticeable hehehe}