Oh man, you’ve got me started now… Brace yourself, here comes a ramble.
I think it has a lot to do with the same reason why people are so drawn to Emily. When people think of Emily they think “sweet, kind, forgiving, seeing the good in people, etc.” and she’s like that with pretty much everybody.
When people think of Spencer they tend to think nearly the opposite. Spencer is extremely sarcastic, holds grudges and tends to see the worst in people.
It shows the differences in how they were raised. Spencer was raised to be cynical and pessimistic because that’s simply the Hastings way, but she is a good person. Emily is like the embodiment of what people think of when they think “good person” and I imagine Spencer feels as though that’s like painting a huge target on her back saying “hurt me”.
People can be cruel. They can manipulate you, blackmail you, use you and all sorts of things. Emily’s not naive, but I imagine (to a degree) Spencer thinks of her as such. Too nice for her own good.
Spencer takes it upon herself to be the one to stand up and fight for her friends, even when they’re willing to let something go. She’s the type of person that once you get close to her, she’ll fight tooth and nail for you. Even if you don’t want her too. She’s stubborn and protective of those she cares about. To the point that she fights with them about it.
And boy does she fight. Just how she tends to be more protective of Emily than any of the other girls, she also tends to fight with Emily more than any of the other girls. She’s a control freak (I say this with love) and believes she’s in the right. Their fights pretty much always stem from her trying to protect Emily in some shape or form. (Any argument about Ali. Even the argument when they were touring colleges was Spencer trying to remind Emily that she was a “good person” and good people don’t use others. She didn’t want Emily to become a “bad person”)
Emily is one of the few people who truly fights back though. Spencer even referred to herself as a “human bulldozer”. But Emily fights back against it. She spent so much time living under Alison’s thumb when she was younger that once she flourished she refuses to bend for anybody unless its something she truly wants to do.
In their arguments, neither one of them is wrong per se. When you get to the bottom of it, they’re fighting because they care so much about each other. I think Emily really has a special place in Spencer’s heart. I don’t imagine she’s ever met anyone else as genuine as Emily. In turn, Emily has admitted before that she likes “ballsy women” and I’m sure that also applies to friendships. (Emily doesn’t seem as close with Aria as the other girls and I wonder if its because Aria’s more passive.)
Marlene King herself has said that while they’re just friends, they’re soul mates in a way. They fit each other well and the other’s opinion of them means a lot to them. The other’s well being means a lot to them. (Remember Radley and the pills and how upset Emily was? How desperately Spencer wanted Emily to forgive her and be there for her?)
Whether you ship them as friends or romantically, you can’t deny that Spencer and Emily have amazing chemistry and they clearly love each other. They’re the type of friends that are forever. (Not to mention the fact that they were the only ones who really stayed in contact and remained friends after Alison disappeared and Aria moved away.)