I overthink things. I split if I feel vulnerable. If I give a piece of myself and realize I may be hurt as a result, even if it's a 1% chance, I get paranoid, and mentally prepare myself to lose the person. So I split them black, get annoyed with their quirks, think the worst of them, jump to bad conclusions, assume they are betraying me or talking bad behind my back, etc. But it's all because I'm afraid they will reject or abandon me. Sometimes in the process I will find or create a reason to end a relationship with them. Whatever that reason may be. And in the process, I wholeheartedly believe in that reason I created. It can be something as simply as them being too friendly with a member of the opposite sex. I'll convince myself that they are flirting or doing something behind my back and I'll emotionally paint them black, just itching for an argument, something to justify pushing them away. Then when I'm all alone and realize that my mind fooled me again, I try to let them back in, but either they're gone or when they come back, it doesn't return to normal. It's almost always related to vulnerability. Say I tell the person I have a mental illness (Bipolar Disorder)... they may be accepting, if I am to take them at face value. But then my mind tells me "they accept you now, but if a guy that has everything you have WITHOUT mental illness comes along, they'll dump you for him". And so, the cycle of negative thinking begins and the thoughts and my paranoid translation of the person's behaviors form a negative feedback loop.
In fact, I have two friends who I keep at arm's distance. I keep everything superficial with them. I don't particularly have any strong feelings for them, I just pass the time with them. I've gone to their weddings and anniversaries, to their engagements, etc. I've been friends with them for well over a decade. And it works because I don't give a ****, really. It works because it's a very surfac-ey friendship. My best friend and I have been literally joined at the hip for 8 years, with a 1 year gap in between where we were not on speaking terms whatsoever. We've since learned to keep certain boundaries in our friendship as we both are vulnerable and sensitive people. I have another close friend who I think has some natural form of Xanax flowing in his veins because this guy NEVER gets riled up. He is the most unassuming, laid back person ever, and he doesn't judge anyone. Another close friend of mine is a female coworker. We've gotten close and she's in AA and we both vent to each other and spend time together. I have a couple other friends that I see every now and then, then I have jamming buddies, then I have acquaintances and coworkers. But, I've had 3 or so close friendships I ended or lost within the past several years. And it's because of vulnerability.