Why is my ex so sensitive?

So I started dating a guy before the summer for around a month. It was really intense and I liked him a lot. But the college semester ended and a few weeks into the newly established "relationship," which had to be long distance for the summer, we ran into problems. He was neglecting me. He would forget to call. Through the course of all this, I was sort of miserable. I made out with someone at a party while we were apart. Drinking was involved and I don't remember anything. I've never done that before, but told him immediately and visited him the next day to talk. I understand he has a lot of work and his family is depending on, but he told me that I will never be a priority. He also told me I was "cold" to him. And after a fight about his "trust" issues and my supposed "intimacy" issues, he said he needed a week to think things through. He also told me he didn't have strong feelings for me. I knew what was coming so I did him and myself the favor of proposing a break until the fall. He was very cold about it though, which he explained was to not give me "any hope." Then he said we'd see each other again in the fall, which I ignored.

Lately, we've been keeping in contact through facebook once a week or so. We seemed to be on amicable terms. He even told me to visit him (which I will NOT do). Yesterday, I teased him about finding out some interesting information on him (from a friend who happened to know him by name) and he came off very sarcastic and called me "very rude."

What is the deal? Why is he being so sensitive? He's the one who wanted to end things. I told him I would've worked things out. He didn't want to try. I like teasing people, and he's just so sensitive. I feel like I'm stepping on eggshells and we've been talking through facebook, not even the phone. We're not even dating anymore. Why is he like this?
Why is my ex so sensitive?
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