Of course, on other days she might only be thinking about all the things about him that turned her off (e.g. he was too smothering and needy, he lacked confidence in himself around her and others, he didn’t know how to keep creating a relationship dynamic between them that 
would make her want to treat him well, be attentive, loving and affectionate towards him).
This causes her to think things like, “What was I thinking! Why did I ever unblock him? He’s all wrong for me.”
Naturally, this might leave a guy feeling confused and asking “Why does my ex girlfriend keep blocking and unblocking me?”