The difference between dumb and stupid

It's Wednesday and my friend A is visiting.  He's waiting for his class to start and is killing some time.  We haven't seen each other in a month, or so, so we're playing a bit of catch up.  Even though not all of the stories are about us, most were headscratchers.

To bring my friend up to speed, I remind him about X, Y and Z.  X and Y were seeing each other long-distance and Y discovered X was in a relationship with Z on Facebook.  There were no explanations or excuses.  Unsurprisingly, it was over between X and Y.  Two years pass and Y receives several messages from X, asking for forgiveness.  As it turns out Z was lying and cheating on X throughout their relationship (talk about karmic irony).  Y decides to give X another try and finds out that X went back to Z right after their reconciliation.

"So, he leaves this one guy who liked him and would've treated him really well and leaves him for another who repeatedly treated him like crap," I explain.

"And he did this twice to Y?" asks my friend.

"Yup."

"Hmmm," he drags out the word.  "Is he that stupid?"

"I don't think he's stupid.  I think he's just dumb."

"What's the difference?"

"That's hard to say.  I believe a person is dumb when they do something wrong and don't even realize it's wrong.  Stupid is when you do it on purpose."  This explanation makes sense, in a rather simplistic way.  "I don't think he did that [the two-timing] on purpose, like he wanted to hurt [the cockholded guy].  I also don't think he knows the difference.  Unless, of course, he's a sociopath.  But, I don't think he is.  So..."

In fact, I'm starting to wonder if Y is the stupid one.  He went through the same situation twice.  But, did he know it was going to happen?  Probably not.  Was there an inkling it would happen, again?  Probably.  He was screwed over once, and with that level of mistrust, he should've learned from his mistakes.  Sadly, he didn't.