Okay so here's what went through my mind.
Long story short: caught my ex making dinner plans with dude via text while I was sitting right next to her, so ties were cut. I was hurt, betrayed, all that. So when ex got dumped by ol'boy I got a measure of revenge because she learned what it felt like. From then she and I had something in common. Because I felt empty and she fit perfectly into the spot she left in my life, I fell back with her and after that we were an on-again, off-again thing.
Also she has AMAZING boobs, so I keep going back for that.