it's probably a random dream. if you were really close with someone and they just left without a real reason, you could still be confused subconsciously and wish to figure it out. hate to ask what happened, but if he just randomly dumped you, you could be wanting recognition as to why and maybe an apology.
i was with a guy for 3 years [really close friends for 2 and a half, dated the other half] long story short, he sent me a nasty text one day at work basically saying we were never really dating, and that he was now going out with our friends girlfriends [meaning my ex stole my friends girl] it's been 3 years now, i've moved on, haven't seen or heard from him since, and have a wonderful boyfriend. for some reason around the time my ex dumped me i have dreams about him saying sorry for what he's done. i don't want him back, i just want him to say sorry.
it could be a similar thing with your dream. if your ex randomly left you without a real explanation, you could just be wanting to have one. you've moved on, but your subconscious mind might still be trying to work it all out. suckie as that sounds, if you;re still in contact with the guy, ask him why he dumped you, or you could try meditating, hopefully you can resolve it yourself. you can never fully escape your past, best to accept and move on. you might occasionally have a dream, but don't think too much into it.