Why am I JEALOUS of my old crush?

I used to really, really like this boy. I mean, I doted on him, I even took him to the formal dance (he had never had a date before as he is sort of a nerd). He hurt his leg pretty badly during one of our plays (he needed surgery) and I was the only friend to call him daily and talk to him to keep his mind off of things. My friends would comment that we were in a relationship, without a formal status. I really liked him for several months, I believed he liked me too- we hung out all the time, and rode to school together (he lives on my block), and talked all of the time (we were also in a play together). He told me he had a great time at the dance (he called it a date first), and he seemed to genuinely like spending time with me, but when I tried to take it a step further and establish a formal relationship, he withdrew quickly and stopped talking to me, no more telephone calls or lunches together, nothing, which I figured was the best for me to get over him. Of course I was heartbroken, I would go to sleep at night with my face wet with tears, I was torn about it. However, in time my affections began to shift as I began liking a new guy, whom I also treated well. He was the German exchange student who moved away over summer, and even though I liked this exchange student during prom time, I was jealous of the old guy having a date, and it confuses me as to why. Now, it seems, while we're on a talking basis, the two are becoming a bit more serious because they went to homecoming together. I know that she is much less pretty than I, and not quite as smart, and I also feel that I am over this guy. WHY AM I SO JEALOUS OF THEM?