Why does my ex only talk to me when she wants to?

I think the title of this thread explains it all. My ex, my first girlfriend ever..and the girl so far that I've really felt for out of all my past girlfriends. Well, our relationship/friendship kinda ended very badly. Once we broke up, we started drifting very quickly. I tried my best to keep it from happening but it was as if she either didn't care or wanted it to happen because it was the best for both of us or something. I admit I was a jerk during a lot of the drifting because I figured..hey if I'm a jerk at least that would give her a reason to talk to me again. If I just accepted our drifting fate, I doubt we'd ever talk again. Anywho, years and years have gone by with nothing but no talking and bad emotions. I'd say for the past 3-4 years she'd if I'd text her there would be a 50% chance of her responding or now. As time went on, the chances of her responding become lower and lower. Eventually I gave up and moved on. Then she starts texting me randomly maybe once every 5-6 months. She'd say things like "I miss the old days" or "how are you". She kinda sounds like she actually cared about me and of course I'm just shocked that she thought about me so instead of saying F you and never talk to her again, I text her back, admitting I've missed her too..etc..even if it was as friends. Why does she do this though. We dated so long ago that it kind of seems silly to think much of it now.

I don't know, she's bored, she's lonely, she wants to know if you miss her, she actually cares about your well-being and wants to know you're still alive, or she just wants to screw with your head. If her contacting you bothers you, then block her.

Edit: You should just block her because this is obviously is messing with your head since you're posting about it here, all confused. Block her, be done with it.